I just did some basic checking on Pricewatch and this is what I came up with:
(all prices USD)
Dual setup w/IDE -
Supermicro P4DCE+ Dual Xeon board: $425
2x 1.5GHz Xeon: $233 each
2x 256MB ECC DRDRAM: $94 each
100GB WD1000JB w/8MB cache: $250
Total: $1329
Single setup w/SCSI -
Asus P4B266 P4 DDR board: $157
1.5GHz Pentium 4: $173
512MB DDR SDRAM: $145
Adaptec 29160 SCSI card: $226
Seagate Cheetah X15-36LP: $495
Total: $1196
So it's actually cheaper to go single proc with SCSI, but I don't think you would see much of a speed boost going with the SCSI drive. I've been using dual proccessor machines for a while and it's pretty nice not to have much of a slowdown when running multiple apps. So my advice would be to go dual, especially if you run several proccessor hungry apps at the same time.
For the people who are going to tell me the Western Digital 8MB cache drives can't begin to compare with the 15K SCSI drives: Yes, the 15K drives are faster, but you get a lot more space for the money. And the speed isn't that bad either. Check out these reviews from Storage Review (for as long as they last anyway.):
http://www.storagereview.com/articles/2 ... -SE_1.html
http://www.storagereview.com/articles/2 ... 0JB_1.html
The WD drives give those 10K SCSI drives a run for their money.