Well that all depends on a few things. First off, RAID stands for Redundant Array of Inexpensive Disks. There are several types of RAID, and depending on what you want to do they may or may not be good for you.
RAID 0 (stripe) is taking 2, or more physicaly different disks, and "merging" them together to form 1 disk. The data is written to and read from all disks at the same time, thereby increasing speed. This would be helpfull in loading the levels in a larger game such as Crysis, etc. The downfall to that is, if 1 of the drives fail, you are screwed. All your data on the drives is lost. Restore from backup. The other "downfall" is that your RAID array is only readable from the particular controler that created the array. So you can't pop your drives into a different tower and expect them to be readable.
RAID 1 (Mirror) is just what it sounds like. You have 2 identical drives, and your controller reads and writes the same data to both drives. Bad thing, twice as expensive, good thing, if 1 drive fails, you're still able to work, game, etc. When you replace the drive that is failed, the controler will rebuild the array automaticaly.
RAID 5 Raid 5 is really the entry point of "Higher End" RAID systems. What Raid 5 does, is stipe data across drives, but it also includes Parity information on the other drives. What this parity data allows is for the controler to contain the data of 1 of the drives in the array. RAID 5 needs a minimum of 3 identical drives to function, and will allow you to operate even when 1 drive fails. If 2 drives fail, you are screwed as well. In a 3 disk RAID 5 Array you will have Disk1, Disk2, and Disk3. Disk1 will contain Disk 1 Data as well as parity information on the Disks 2 and 3. Disk 2 will contain Disk 2 Data, and parity for disks 1 and 3, and Disk 3 will contain Data from Disk3, and parity from 1 and 2. So you see how if one of the drives failes, the other 2 have enough information in Parity to not only keep the array functioning, but when you replace the disk, the other 2 drives will rebuild the failed disk.
RAID 5 will typically give you roughly 2/3 actual usable capacity compared to what the full drive count is. 3 100Gb disks will yeild approx 200GB useable drive space.
That is typicaly where the "average" user will stay in regards to RAID Levels, with RAID 5 being less likely due to the cost of the disks. Now if you were a home business for example, RAID 1 (Mirroring) would be good for you. But if you wanted the benefits of Data Redundancy AND speed, then RAID 5 would be your best bet.
As it pertains to Gameing, I would recomend just staying with the single drive, and always, always have a backup solution for any "critical" files. Be it, copying them to DVD/CS or something like that.
Edited for spelling mistake.