If you're absolutely certain that's all you need:
OS: Windows 7 Home Premium OEM - $99
Motherboard:
ASUS P8H1 LX PLUS - $70
Processor:
Intel Pentium G860 - $100
RAM:
G.Skill Ripjaws 2GBx2 DDR3-1600 - $30 (This memory should clock down to DDR3-1333, as that's the most the motherboard can support, if it doesn't, well we should stop recommending G.Skill

)
GPU:
EVGA GeForce GTX 550 Ti - $135
HDD:
WD Caviar Blue 500GB - $75
Total: $510
If you need a case:
Antec NSK 4428B - $95. It comes with a 350W Antec power supply
If you're really short on cash, I think you can run D3 on an AMD A8 APU, but at reduced graphical settings.