I'm trying to find a $500 gaming build.

Here's my recommendation :

PCPartPicker part list: http://pcpartpicker.com/p/1z5pv
Price breakdown by merchant: http://pcpartpicker.com/p/1z5pv/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1z5pv/benchmarks/

CPU: AMD FX-4130 3.8GHz Quad-Core Processor ($97.98 @ Outlet PC)
Motherboard: Gigabyte GA-970A-DS3 ATX AM3+ Motherboard ($79.99 @ SuperBiiz)
Memory: G.Skill Sniper 8GB (2 x 4GB) DDR3-1600 Memory ($58.50 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ Newegg)
Video Card: XFX Radeon HD 7850 1GB Video Card ($114.99 @ Newegg)
Case: Zalman Z5 Plus ATX Mid Tower Case ($39.99 @ Microcenter)
Power Supply: Corsair Builder 500W 80 PLUS Certified ATX12V Power Supply ($29.99 @ Newegg)
Optical Drive: LG GH24NS95 DVD/CD Writer ($15.99 @ Microcenter)
Total: $497.42
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-09-02 11:58 EDT-0400)