PCPartPicker part list:
http://pcpartpicker.com/p/UDY8
Price breakdown by merchant: http://pcpartpicker.com/p/UDY8/by_merchant/
Benchmarks: http://pcpartpicker.com/p/UDY8/benchmarks/
CPU: AMD Phenom II X4 965 Black 3.4GHz Quad-Core Processor ($84.99 @ Newegg)
Motherboard: ASRock 960GM/U3S3 FX Micro ATX AM3+ Motherboard ($60.97 @ Newegg)
Memory: A-Data 8GB (2 x 4GB) DDR3-1333 Memory ($61.73 @ Amazon)
Storage: Hitachi Travelstar Z7K500 500GB 2.5" 7200RPM Internal Hard Drive ($59.99 @ Newegg)
Video Card: Gigabyte Radeon HD 7850 1GB Video Card ($179.99 @ Amazon)
Case: Cooler Master Elite 311 (Blue) ATX Mid Tower Case ($45.15 @ NCIX US)
Power Supply: Raidmax 500W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($43.99 @ Amazon)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer ($17.98 @ Outlet PC)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.95 @ Amazon)
Total: $644.74
(Prices include shipping, taxes, and discounts when available.)
This should pretty much handle any game you throw at it. The PSU is a little overkill but it's good for upgrading your GPU/CPU.