This is about $40 more than your original build, but will game really good. you can always add an ssd and upgrade the CPU later.
PCPartPicker part list:
http://pcpartpicker.com/p/y5pN
Price breakdown by merchant: http://pcpartpicker.com/p/y5pN/by_merchant/
Benchmarks: http://pcpartpicker.com/p/y5pN/benchmarks/
CPU: Intel Core i3-3220 3.3GHz Dual-Core Processor ($129.99 @ Newegg)
Motherboard: ASRock H77M Micro ATX LGA1155 Motherboard ($69.99 @ Newegg)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory ($47.99 @ Newegg)
Storage: Western Digital Caviar Blue 500GB 3.5" 7200RPM Internal Hard Drive ($59.00 @ Compuvest)
Video Card: XFX Radeon HD 7850 2GB Video Card ($169.99 @ NCIX US)
Case: Zalman Z9 ATX Mid Tower Case ($31.98 @ NCIX US)
Power Supply: XFX 550W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($53.99 @ NCIX US)
Optical Drive: Asus DRW-24B1ST/BLK/B/AS DVD/CD Writer ($19.98 @ Outlet PC)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.98 @ Outlet PC)
Total: $660.89
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-01-17 13:17 EST-0500)