Well here's a setup for $800:
Mobo: MSI K8N Neo4-F $80
CPU: AMD Athlon 64 3700+ San Diego Core $223
Video Card: EVGA GeForce 7800GT 256MB PCI-E $275 after rebate http://www.newegg.com/Product/Product.asp?Item=N82E16814130247
Optical Drive: DVD Burner $50 or less
Hard Drive: Not really my specialty (Allow $100 or so)
RAM: 1GB value RAM $80
Total: Comes to $808 (w/ $50 DVD burner & $100 hard drive)