Well, in theory, a good SCSI controller and top notch SCSI Drive will readily outperform the current best ATA drives -- some trade offs:
Cost -- ATA drives are a lot cheaper as you move toward the upper end (ie 80G and up) relative to SCSI.
Capacity -- the best price point for SCSI drives is 36G 15K (which is going to perform VERY well indeed) -- as you go to 72G the cost soars and it is harder to track down the 15K drives.
Now if you were running a server, SCSI would be the way to go, in large part because the way a SCSI controller/drive combination offloads CPU traffic from the primary CPU, and also because the SCSI controller/drive combination is much more efficient at handling multiple concurrent read/write requests (as in network use).
I use a 40G ATA-100 drive internal, and an 18G SCSI 10K U2W (80M/Second) drive externally. The real reason for the SCSI drive -- easier for me to swap to another computer downstream (it has my support images and music). Also, that external SCSI drive is set to be shared out in the home network (the Music is available to any computer in the home office).
One other payoff with the SCSI drives -- you can not only go external (because the cabling support allows for longer and external chains), but also you can chain many more drives). And because it is external, you don't have a power drain or heat build up in the case. Those 15K SCSI drives do run rather warm.