$500 is a very low budget. So if u wanna get a $500 rig, I'm sorry but it has to be done to cut the HD 7870 to HD 7850 to keep the budget down.
PCPartPicker part list:
http://pcpartpicker.com/p/1lDrJ
Price breakdown by merchant: http://pcpartpicker.com/p/1lDrJ/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1lDrJ/benchmarks/
CPU: AMD FX-6300 3.5GHz 6-Core Processor ($109.99 @ Amazon)
Motherboard: Asus M5A97 LE R2.0 ATX AM3+ Motherboard ($79.98 @ Outlet PC)
Memory: Corsair XMS 8GB (1 x 8GB) DDR3-1600 Memory ($49.99 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($69.99 @ Newegg)
Video Card: Sapphire Radeon HD 7850 1GB Video Card ($129.99 @ Newegg)
Case: Thermaltake Commander MS/I Snow Edition (White/Black) ATX Mid Tower Case ($34.99 @ Microcenter)
Case Fan: Rosewill RFA-120-BL 74.5 CFM 120mm Fan ($4.99 @ Newegg)
Power Supply: Corsair CX 500W 80 PLUS Bronze Certified ATX12V Power Supply ($34.99 @ Newegg)
Optical Drive: LG GH24NS95 DVD/CD Writer ($14.94 @ Newegg)
Operating System: Microsoft Windows 8 (OEM) (64-bit) ($89.73 @ Outlet PC)
Monitor: AOC e2351F 60Hz 23.0" Monitor ($117.47 @ TigerDirect)
Total: $722.10
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-07-28 15:16 EDT-0400)
Don't be too stingy to PSU, a semi-modular PSU does good to stabilize the system and sustain more heat.
It's better if the budget is $600 it'll be more stronger build based on FX-6300. I could see u've done quite a lot of research on that.