Mobo/RAM: Gigabyte GA-P55A-UD3 and Corsair XMS3 2x2 GB 1600 mhz CAS Latency 8
$220. Not my favorite sticks of RAM, but the combo is really good.
HDD: Seagate 7200.12 500 GB
Case/PSU: Antec 300 Illusion and Neo Eco 520W
Optical: Cheap SATA DVD burner
GPU: HD 5450
Total: $669. You can maybe stick in a SSD to speed the boot and loading times.