don't mess with the dual xeons, I just built the following for a customer and man is it a killer
Dual Opteron 244
2048 ecc-reg pc3200
4 x raptor 10k s-ata (raid-5 w/hotspare)
3ware 9500 (128m cache)
3u rackmount
$1600
that will beat the pants off any xeon you can build, check out
www.chiefvalue.com if you really want cheap prices and great turnaround on orders.