You don't have to get RAID. In fact, SATA RAID can be a fairly risky thing. The connectors in the SATA standard are a bit suspect and drive failures have occured over this.
Lots of gamers and other power users will commonly employ use of a RAID 0 scheme where two drives essentially become one. You get nearly double the disc I/O speed from the configuration, but you also get double the risk as well. If either drive fails your data on both drives is toast.
Many of the hard disc drives on the market are very fast. Most hard discs in new desktop machines typically have 7200rpm speeds and at least 8MB of cache. Some even go double to 16MB. Notebook hard discs have also been getting fast. Very fast external hard discs are also available. I'm planning to get a 500GB external disc for my notebook and connect it via Firewire 800.
Here's my recommendation for hard discs in a desktop machine:
Use at least two physical hard discs. But don't automatically hook them up in RAID configuration. Keep the boot hard disc in a normal setup. That's going to lessen the wear and tear on the drive and lessen risk to data on the other drive.
For hard disc #2, hook it up on a separate SATA channel, or even better attach it to a separate controller.
Windows won't be able to monkey around with it easily. You'll get better scratch disc performance from applications like Photoshop. Other tasks like video editing will run more smoothly.
IMHO, RAID only needs to come into play for very specialized purposes. RAID 1 (mirroring) makes sense for a data server. RAID 0 works well for professional audio editing (such as a complete ProTools setup), HD video editing, data servers for video and other stuff like that. SATA isn't the best format for it though. In professional environments U320 SCSI RAID or Apple's XServe (FiberChannel) RAID are more stable choices. Be prepared to get out your wallet though.
About the only way I would feel comfortable doing SATA RAID in a desktop
computer setup is by using 3 hard discs, one separate boot disc on one controller and the other two drives in RAID 0 on another controller. The RAID setup would only be used for temporary use -such as Photoshop scratch discs, HDV video capture, etc. I would back up captured video to external hard discs.