I've had the benefit of being behind two days in my email when I hit this thread, so I got to see the entire thing evolve. Here's my two cents. Of the remaining two bounds, the one that matters really starts depending on what you're doing. That takes CPU power, and memory for those proccesses to reside in. Someone posted about using a RAID, and I agree on that for speed and reliability issues.
Several drives working in conjunction are faster than just one.
You also want a fast SCSI controller. Consensus on this list seems to be either the Adaptec 2940 or the Buslogic 946c, both of which run on a PCI bus. Some RAID controllers use SCSI-IIFW, though. One thing that only one person really mentioned is that the database lookup took 12 seconds on average. Look at your cgi-bin code and be sure you're using the most efficient algorithm available to you.