PCPartPicker part list:
http://pcpartpicker.com/p/3pa6R
Price breakdown by merchant: http://pcpartpicker.com/p/3pa6R/by_merchant/
Benchmarks: http://pcpartpicker.com/p/3pa6R/benchmarks/
CPU: AMD Athlon X4 750K 3.4GHz Quad-Core Processor ($79.98 @ OutletPC)
Motherboard: MSI FM2-A75IA-E53 Mini ITX FM2 Motherboard ($84.98 @ SuperBiiz)
Memory: Team Elite 8GB (2 x 4GB) DDR3-1333 Memory ($69.98 @ OutletPC)
Storage: Seagate Barracuda 500GB 3.5" 7200RPM Internal Hard Drive ($58.40 @ Amazon)
Video Card: Gigabyte GeForce GTX 750 Ti 2GB Video Card ($149.99 @ NCIX US)
Case: Rosewill RS-MI-01 BK Mini ITX Tower Case w/250W Power Supply ($48.98 @ Newegg)
Total: $492.31
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-04-09 22:37 EDT-0400)
This is slightly over your price range, but can play most games on medium-high settings on a playable framerate. It can also be used as a steam-box if you would like. Also, the case comes with a pre-installed PSU.