Coming from a user of both RAID and high performance hard drives, both RAID and Raptors are a waste for a gaming machine, gaming is the last thing you need a fast hard drive for. Once the game is loaded into the RAM, there's very little reading off of the hard drive. You won't get faster fps with a faster hard drive.
But if you have $400 to spare for storage I recommend 2 150 GB Raptors in Raid 0. Cant be beat except my Cheatahs...Plus they are damn sexy in a lit up open case window at Lan parties, other folks drool over them.
If I had to rebuild my computer, knowing what I know now, the Raptor would be the only part I'd buy again. It's one of the better performance updgrades I've done. Load times, defrags, installs, etc. are all greatly improved.
That being said, don't buy one if you don't have the cash. If you only care about gaming, then you should get a 7200 drive, and use the ~$200 you save for a better video card -- you'll get better bang for your buck that way...
get two drives. buy the smallest possible raptor (i believe it is 36 gigs) and install your os and any apps that need a speed sensitive hard drive. install everything else on a standard 7200.
the downside to the raptors is they sound just about like coffee grinders at times...they can be loud! if you are one of those people who gets angry if you can hear your machine over the noise of small insects, you don't wanna go with a raptor.