Well using
Centre Com prices I think you're looking at something like this:
Gigabyte GA-P31-DS3L motherboard: $119.90
Intel Core 2 Duo E6600/ 2.40GHz/ 4MB Cache/ 1066FSB/ LGA775: $284.90
Kingston DDR2 2GB PC-5300/667 (2x1GB) CL5 Value Ram: $69.99
Western Digital 320Gb 7200RPM 16Mb SATA 2: $93.00
ASUS EN8600GT-HTDP-512 GF8600GT, 512MB, DDR3, PCIEx16, HDTV, 2xDVI-I, HDCP: $192.00
Total: $759.79
You might also want a new case and possibly a DVD burner.