MOBO- Cheap, yet nice-http://www.gameve.com/gve/store/productdetails.aspx?sku=MB-ABIT-037
CPU- AWESOME in BOX w/fan+heatsink AMD64 3200+-http://3btech.net/amdat6432so9.html
Ok, that is about it. GRAND TOTAL- 340 dollars or so. That leaves you 360 dollars to get whatever. Do you have DDR PC3000 or 3200? If not, you will need that. Got a good graphics card. Tell me what else you really need, and Ill post that below.
Or try www.cyberpowersystem.com
and click AMD64 configurator for a system that starts at $512, no joke. Your choice.