Since I have both SCSI and IDE in my system, I guess I should throw my two cents in. I can't tell the difference. The newer generation of IDE drives can be quite fast, and I just don't see a performance hit when using IDE vs SCSI. This is with kernel compiling, ripping MP3/OGG, etc.
SCSI is great for lots of things, but in a desktop machine I say save some money and go IDE. The marginal improvement you may get on a benchmark is not worth the money. Note: I did not pay for my SCSI drives; they, uh, fell off a truck.