Raids, Faster than 7200RPM (5400RPM I guess) drives, SCSI all won't have much of an impact on gaming except loading times. Like for DOOM III and Farcry you will cut down on loading times though. Also those would be good for getting windows to startup really fast. If I were to get a drive I would get a 15K SCSI drive as boot and a large capacity drive (mostlikely IDE) for storage or game installs. Or I could raid two scsi drives together and then have it as boot drive and for game installs, the large capacity IDE drive would be for storage of movie files, music etc.
The reason why it wont likely improve performance much is because games these days all load to the ram, unless your low on ram there wont be much performance improvement. I heard that there are some ppl who have played games with 128MB of ram or had thier system so that it was continuously pageing to the fast SCSI drive and it ran surprisingly smooth (according to them). I would love to see that happen! Run farcry with like 64MB of ram! OR maybe 32! HOW LOW CAN YOU GO!