I got a compatible 475W PSU on eBay for $50, and I think I can foot the $175 for 512MB of RDRAM on ebay. Now all that's left is an Extended-ATX case....this thing is a beast!
The video card (to be) is a 6600GT 8X AGP, 128MB G-DDR3. I think that I may just build this as a secondary rig, since if I used this instead of the athlon I'd have to replace the case, hard drive and power supply too. I feel sorry for my bank account.
But still, how will the Xeon do against the Athlon in everyday performance?