PCPartPicker part list:
http://pcpartpicker.com/p/1peqm
Price breakdown by merchant: http://pcpartpicker.com/p/1peqm/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1peqm/benchmarks/
CPU: AMD A10-5800K 3.8GHz Quad-Core Processor ($109.99 @ Amazon)
Motherboard: ASRock FM2A55M-DGS Micro ATX FM2 Motherboard ($49.99 @ Amazon)
Memory: Corsair 4GB (1 x 4GB) DDR3-1600 Memory ($32.99 @ Newegg)
Storage: Seagate Barracuda 500GB 3.5" 7200RPM Internal Hard Drive ($53.98 @ Outlet PC)
Case: Fractal Design Core 1000 MicroATX Mini Tower Case ($39.98 @ Outlet PC)
Power Supply: Corsair Builder 430W 80 PLUS Bronze Certified ATX12V Power Supply ($19.99 @ Newegg)
Total: $306.92
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-08-07 06:16 EDT-0400)
The APU will be able to run all source games easily, and when you have more money you could add a Dedicated GPU.