Okay here's my input.
Mobo:
ASUS A8N5X Socket 939 NVIDIA nForce4 ATX AMD Motherboard $80
CPU:
AMD Athlon 64 X2 3800+ Manchester 2000MHz HT 2 x 512KB L2 Cache Socket 939 Dual Core Processor $300
Memory:
G.SKILL Extreme Series 2GB (2 x 1GB) 184-Pin DDR SDRAM Unbuffered DDR 500 (PC 4000) Dual Channel $160
Grahics card:
XFX PV-T71G-UCE7 Geforce 7900GT 256MB 256-bit GDDR3 VIVO PCI Express x16 Video Card 520Mhz core 1500Mhz memory clock $330
A bad ass gaming system = priceless
That's $870 and I don't know about the rest. But this is a good set up though I can tell you that. I don't know if you have an old case and psu. Or are you building it from the ground up excluding the monitor and other input devices?