Dual Opteron vs Dual Athlon

I'm trying to build a very high-performance, low-cost SMP workstation. I have narrowed down my decision to one of the following:

1. Dual Opeteron 240 (1.4GHz chips) (64-bit)

or

2. Dual Athlon MP 2800+ (2.08GHz chips)

Cost-wise, they are pretty close. I can't seem to find a good comparison of the two. And I can't make up my mind about which way to go. I'd like to find out which of the two will yield higher performance under winxp pro 32-bit. So, basically, I'd like to know what you guys think. I'm hoping that the Opterons will be faster (i.e., 64-bit capability, HyperTransport technology, and faster RAM support is enough to offset the difference in GHz marks). Educated responses only, please.

One more question. Regarding disk I/O performance, I have come to another cross road between:

1. 10,000RPM Seagate Cheatah U160 w/8MB cache on a 64bit PCI LSI Ultra 160 Controller (in a 64-bit PCI slot, of course)

or

2. Four (4) 40GB ATA133 Maxtors w/2MB cache on a Promise ATA133 Raid card in 4-disk-striped array (ala Raid 0)

TIA, for your comments.