First off, Adaptec is the only way to go, IMO. It might be touchy on the clock rate, so if you're running a KT400x, you might want to look at something else. The main advantage is the reduced CPU load, and you just can't beat Adaptec for that.
U320 is too much, for a workstation. I've been looking at U160, and you know what? It's still too much!
Check out HDDs at
www.storagereview.com and look for the data rates. You'll find that an Adaptec AH-2940-U2W will fit the bill just fine. Otherwise, you can do like me, and aim towards a AH-29160, which will run just about anything without any issues.
The trick with the 2940 is that there's more than one of them, and you really have to find the one that allows the 80 MBPS data rates, to accomodate the latest drives.
Are we skipping over RAID?
Have you looked at the prices of this stuff?
BTW, SCSI drives have a number of advantages. Do you know what they are?