Here's what I would pick:
AMD Athlon 64 4000+ San Diego 1GHz FSB 1MB L2 Cache Socket 939 Processor - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16819103529
or Dual Core
AMD Athlon 64 X2 4400+ Toledo 1GHz FSB 2 x 1MB L2 Cache Socket 939 Dual Core Processor - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16819103546
DFI LP UT nF4 SLI-D Socket 939 NVIDIA nForce4 SLI ATX AMD Motherboard - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16813136158
OCZ EL Platinum Revision 2 1GB (2 x 512MB) 184-Pin DDR SDRAM Unbuffered DDR 400 (PC 3200) Dual Channel Kit System Memory - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16820146890
eVGA 256-P2-N527-AX Geforce 7800GTX 256MB 256-bit GDDR3 VIVO PCI Express x16 Video Card - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16814130254
Keep that hard drive you have already. Total price is a coulpe hundred dollars more than your specs, but I like looking at this one. It's not bad. More money with the dual core.