I'm concentrating on latency because, we're talking about applications (database, and to be on topic, a heavily loaded mail server) where it is one of the most important factors. For a heavily loaded mail server, it will be taking many relatively small chunks of data and dumping these to disk as well as reading these from disk. Here, bandwidth is not as important since most of the time will be spent getting the drive in position to read/write data. The latency of a 15000RPM drive will greatly out perform a 7200RPM drive (again, regardless of the interface).

Of course, the interface and bus to the system does make a difference, but this generally doesn't make such a performance impact as the latency does in such applications.

Here's some decent reading on SCSI vs ATA:
http://storagereview.com/...compPerformance.html
http://storagereview.com/.../if/compSummary.html

Adrian