It entirely depends on the SCSI technology you're going with. I upgraded from ATA-66 7200RPM IDE to Ultra160 SCSI with a 15,000RPM Seagate Cheetah drive for audio, and it benchmarks *significantly* higher than the IDE drive. The other guys are right in that the access time is less important for DAW as long as you keep your drive defragmented, but for myself I benefit from the throughput and speed of the SCSI drive.
The hard drive is a classic bottleneck, if you're playing back a lot of stereo tracks while recording simultaneously, then the better your drive is the better off you'll be. You need to look at your situation though, how you'll be recording. You very well may not need the extra performance and can sink the $330 into more RAM. With RAM prices the way they are I wouldn't even consider building a new computer for DAW with less than 512MB (I run 768MB in mine). By the time I'm done with a song I often have 20-30 stereo audio tracks that I'm muting/unmuting and A/B'ing tracks on the fly (40-60 mono tracks), plus another 8-10 MIDI tracks. Dense arrangements can really choke a slower drive. Of course by the time I'm done I have it whittled down to a single stereo track, but that's beside the point.
--Will--