I hate to jump back into the Mac vs. PC debate again, but this is something that I have been speaking about and dealing with literally every day for numerous years now, both in a daily user perspective and as an owner of a computer repair/services business.
First off, I'll start with just the 32-bit vs. 64-bit area. With a 64-bit system you are able to move past the 4 GB of RAM limitation that exists with a 32-bit operating system. In a design system, this is huge. If you're working with large files in Adobe or even in Flexi while multitasking with other applications open (email, internet, media, etc.) then the more RAM you have the better. Anymore, when we're building design systems, I'm going to always suggest 16 GB of RAM and here's why: RAM is cheap, and while most of your applications are never going to use that much RAM regularly, it will help in the end three years from now when you've upgraded your software twice and it's now able to actually utilize that amount of RAM. Plus, you can download free software to create a RAMDisk, which allocates part of your RAM to be used as an actual hard drive. Let Flexi and Adobe (and even Windows if you wish) use this RAMDisk as a temp scratch disk and you literally cannot get a faster system. RAM operates at several hundred times faster than even the fastest SSD on the market meaning instantaneous renders, filter applications, and lightning fast response moving or editing even large files in all your design software.
However, I would not suggest going with Windows XP 64-bit edition. First off, it's going to be hard to find. Second, Windows XP will losing "service" from Microsoft meaning they will not be releasing any security updates, patches, or fixes. It's an old operating system, and while it still runs alright, the 64-bit version can be buggy and if you're going new, go Windows 7. I've been incredibly pleased with Windows 7 as have pretty much everyone been that has upgraded to it.
Now, into the Mac vs. PC part *deep breath*. Now, most of what I am going to cover is pure FACT, and I know saying that right there is going to get me a lot of flack, but I deal with it daily and I've done years of research on it. However, let me be very clear about one thing. In the debate between the two, it ultimately comes down solely to preference. If you've learned and like the interface of a Mac, you're going to choose the Mac. Same can be said about the PC. But, it's important that you know and understand the true differences here before you make the decision to spend tens of thousands of dollars to re-outfit your entire shop to a new platform which will require not just new computer purchases, but all new software, possibly new peripherals (printers, scanners, etc.), and a LOT of time training.
First off, the actual parts that make up a PC and a Mac are 99.9999% identical. The ONLY thing that makes a Mac a Mac is that it has a specific BIOS programmed into it that allows it to boot and run OS X. If you go and configure a Mac Pro computer for design, you can design a Dell Precision which uses the exact same processor, exact same type of RAM, exact same hard drives, exact same graphics cards. Everything. The difference here is that the Mac will cost you up to several thousands of dollars more than an identical PC system (if you're planning to have a custom-built system or build your own, you can save MUCH more than even going with a Dell or HP etc.)
So what makes everyone have this belief that Macs are better? Why have photographers and designers been using them for years? Well, that requires a little history lesson.
Many years ago, during the mid 1990's, computer hardware was beginning to rapidly change and get more powerful at a rate never seen before. Macintosh at the time developed computers that focused on having a more "pretty" user interface and graphical power than many of the available PCs because, at the time, PC didn't really focus in that area. It was at this time that Apple computers began developing the reputation for having "superior graphics capabilities" and indeed, at that time, they did. But after a few years the PC computers available offered just as much graphics power as Macs did. It took a little time for such power to become very mainstream and for software to also accommodate the graphics power now capable within PC computers, but it is definitely true now that PC computers offer just the same graphics hardware and capabilities as found in a Mac. Again, the parts between a PC and a Mac are identical.
A second myth that people often rush for a Mac for is that they are "immune to viruses." It is these same people that I get to deal with in my business because they call me up when their Mac is acting weird. Behold, their invincible Mac has indeed been infected with a virus. Every operating system out there is capable of being infected with a virus, even the OS that runs your DVD player, the computer in your car, everything. The reason that this myth started is because of how OS X was created. In the early part of this decade, OS 9 was designed on its own core or kernel (basically the core of the operating system) much like Windows NT is its own kernel. Macintosh made a drastic change and ditched this original kernel and built their next operating system, OS X, based on the UNIX kernel which has been known for decades as a very powerful and secure OS, easily customizable, and up until that time really only used for large-scale server systems. In its early years, the UNIX based OS X kernel was quite new so relatively secure to existing virus threats. Also, Macintosh controlled a tiny fraction (then about 3 - 5 %) of the market share of operating systems in the world. Why should malware coders design viruses that would only affect a tiny sliver of market out there? Their more likely to succeed by coding for the other 95% of market share in use.
Several years have passed since the OS X kernel was originally formed and unfortunately Macintosh has not made major modifications and updates to the core kernel since it's design. In fact, a great number of leading security analyst experts agree that the new Windows 7 operating system is natively more secure than the OS X kernel, mostly due to the long time periods between patches and updates from Apple compared to the rigorous testing and numerous patch updates put out by Microsoft.
So, if you believe that you can get a Mac and not worry about viruses, I hope you're prepared for some bad news. Macs can get viruses, just the same as PCs. Here's the difference, though. There are less security utilities available for protecting Mac systems than there are for PCs of course. If you aren't running proper Mac compatible anti-virus software and your Mac does get a virus, there is no fix that any computer technician can do to help you. Apple's actual suggested action for a Mac virus is to bring it into an Apple store so they can completely wipe your hard drive and reinstall OS X (and yes, you will lose your data most likely.) However, with a PC if you do get a virus, and get an actual computer technician to repair it, you will not have to wipe your computer and you won't lose your data. They run their scan and fix utilities and you're back up and going. I can honestly tell you in three years of running my business, and removing viruses from literally more than a hundred computers, I have only ever had to reinstall the operating system on a computer 1 time after a virus infection and even then we were able to recover all the data before reinstalling Windows XP.
And that brings me to the next point, one which was made above by others, and that is serviceability. There are a great number of computer technicians out there that are proficient with making your PC work, network with everything in your office, and have drivers and software that are compatible. A Mac computer, on the other hand, will be much harder for any service technician to repair because everything is locked down by Apple as they want only their Apple Store technicians performing any sort of work on their computer. This means limited support of systems compared to PCs (by far!) Additionally, while the hardware within a Mac is identical to that found within a PC, they will charge you a tremendous premium for any sort of hardware replacements or upgrades compared to purchasing an component for a PC and adding it.
Just to give you an example, I recently saw some DDR3 1333 ECC Registered RAM from Apple for use in "upgrades for Nehalem-based Mac Pros". A single 2 GB stick of memory was a whopping $300! That same 2 GB stick of DDR3 1333 ECC Registered RAM from Newegg was less than $75! There are also a lot of components that are just proprietary enough by Apple (such as power supplies) that you must purchase that exact component through Apple for replacement. You can't just put any ATX compatible power supply in for replacement or upgrades, which would save a TON of money.
Now, to end this long rant of mine I just want to specifically state for you and everyone to read that I am not flat out telling you that you should never buy a Mac computer. Again, it all comes down to your own preferences and circumstances. But I do want you to be aware of the facts about the differences between the computers. In the end the simple truth is if you configure and purchase a Mac computer system, you will spend a LOT more just on hardware alone for that computer when compared to an identical PC (whether through a custom builder or a retailer such as HP or Dell.) Slowly, Apple has been getting their prices closer and closer to the similar PC price but it is still a long ways off. The thought that the Mac is far superior in performance or quality is false because the hardware found in both systems are identical. The only thing that is different is the operating system and again, it comes down to your preference to use.
The whole reason for this discussion is exactly as you said, to understand the options available out there. If you're really thinking about getting a Mac computer, it's almost universally suggested that you should switch nearly all your computers to Mac or there will be some compatibility issues undoubtedly between the Macs and the PCs on your network. Additionally, you're going to have to consider your software. Is all your software Mac compatible? If so, will you have to purchase new Mac licensing and specific versions of that software for use on the Mac computers? Will all your additional devices, such as printers, all work on your Mac as well?
If you'd like a detailed, specific pricing break down of upgrading all your PCs to new Windows 7 PCs compared to having to upgrade your whole office to Macs (and possibly replacing additional hardware, plus new software licensing) then I could do that, but needless to say I can guarantee it will cost you a LOT more to switch over to the Mac computers. There are a ton of options out there for going with PCs that are highly customizable and fit your business and your needs ideally. I'd highly recommend speaking with Casey from Signburst PCs as well on some of your specific usages as well as options with custom PCs specifically built and tailored for the design industry.
. That really should tell you something. 
