Really it is all in what you need. Raid can be set up so that you can span two or more drives, or it can be set up to mirror drives (depending on the raid controller options, all are not alike). In raid systems where they span drives the write time is a lot better=faster, but most often the read time is worse=slower. In the old days spanning drives was good because you could keep up with importing huge files from digital video cameras. Now the newer 7200 drives do just fine like they are and they are now big enough that you don't really need to span a drive for the space. Also with raid you have to worry about flakey controllers and drivers. There are a couple good ones out there that offer good controllers and drivers, but sometimes there still may be problems. It is recommended that you use identical drives to set up your raid array. Instead of a raid controller I try to stick to motherboard solutions that offer two additional regular ide controllers. Some raid controllers allow you to have a hard drive installed as a single array, but depending on the controller you may run into problems with more than that. Most of the time you can not hook up cd and cdrw drives to the raid controller.
Hope this helped a little.
Will