This build should play most games on high to max settings. It has room to SLI on the board, but if and when you do so, you will need to get a bigger power supply.
If you have any questions, just ask.
PCPartPicker part list: http://pcpartpicker.com/p/kfMX
Price breakdown by merchant: http://pcpartpicker.com/p/kfMX/by_merchant/
CPU: AMD Phenom II X4 965 Black 3.4GHz Quad-Core Processor ($89.99 @ NCIX US)
Motherboard: ASRock 970 PRO3 ATX AM3+ Motherboard ($77.55 @ Newegg)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory ($39.99 @ Newegg)
Storage: Western Digital Caviar Blue 500GB 3.5" 7200RPM Internal Hard Drive ($55.99 @ NCIX US)
Video Card: XFX Radeon HD 7850 1GB Video Card ($159.99 @ Newegg)
Case: Cooler Master Elite 430 ATX Mid Tower Case ($29.99 @ Newegg)
Power Supply: Corsair Builder 430W 80 PLUS Certified ATX12V Power Supply ($34.99 @ Newegg)
(Prices include shipping, taxes, and discounts when available.)
Note: Cost is $538 before rebates.