Just remember software compatibility.  ... If you're running older  software and it didn't pass the xp to vista compatibility issues ...  you'll have to buy new software.  I think for adobe that was cs2? and  older if i'm not mistaken.  Otherwise, the 4 things that make a halfway  decent design 
computer on a budget is 1. the ram ... you have to be able  to buffer all that data so it doesn't gum up trying to open or use  files.  2. the video card ... same as ram ... it has to be able to show  you what you are doing so you don't get phantom movements. 3. the  storage ... nothing says fubar like filling up a HD with revisions,  proofs, exports and other b.s. when you need to save a large digitally  painted illustration. and lastly ... 4. the monitor ... I can't describe  how important it is to have a decent monitor. I've seen shops running  demon machines that can slap your mother and make you a sammich at the  same time .. but they are running 15" crts still.  make sure 25-40% of  your budget goes into dual monitors with a decent size and aspect  ratio.  
Lastly, if you have $3k for 3 systems, that is easy to get awesome machines that have what you want.
Make  sure your computers you are getting have 4 ram slots makes it way easier to upgrade ... office depot  will open them up just to make sure and are about 25% cheaper than alot  of other places.  you can get a decent machine for about $300-500, throw about $150 worth of ram into it, and get a couple decent monitors for about $300-400 ... done.  I did this and now I'm running a quad core machine with 16 gigs of ram, a 4 tb hd with a most excellent video card (some other frivolous b.s. like a blue ray drive) and two 22" monitors.  I've also since upgraded my main design 
computer to have 2 additional 2 tb firewire hard drives (one backup another for general storage) upgraded the video card to a radeon with 2gb of memory and installed 2 spare cards worth of usb ports.  (total of $300 upgrade and 2 hours of my life to clean the case, install the cards and set up the new drivers
Unless you are doing some serious compiling and rendering for 3d design, that machine will whip your designers asses in reliability and longevity.  And I almost assure you your designers won't use nearly half the system they think they need.  Especially if they do just vector graphics.  I have a few friends who have top of the line machines, but they also do a ton of 3d modelling so it pays to have the best. 
plus, you get too good of a machine you spoil your employees.   Either that or they secretly install black ops on them networked together because they are kick *** machines ... can't have that.