A bit too hard to keep the price under $400, but managed to keep it very close to it. Here's my recommendation :
PCPartPicker part list:
http://pcpartpicker.com/p/1CxPl
Price breakdown by merchant: http://pcpartpicker.com/p/1CxPl/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1CxPl/benchmarks/
CPU: AMD Athlon II X4 750K 3.4GHz Quad-Core Processor ($79.99 @ Amazon)
Motherboard: MSI FM2-A75MA-P33 Micro ATX FM2 Motherboard ($49.99 @ Microcenter)
Memory: Crucial Ballistix 4GB (2 x 2GB) DDR3-1600 Memory ($30.83 @ NCIX US)
Storage: Toshiba 500GB 3.5" 7200RPM Internal Hard Drive ($44.99 @ Newegg)
Video Card: PowerColor Radeon HD 7850 2GB Video Card ($129.99 @ Newegg)
Case: Fractal Design Core 1000 USB 3.0 MicroATX Mid Tower Case ($26.99 @ Microcenter)
Power Supply: XFX 550W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($44.99 @ NCIX US)
Total: $407.77
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-09-11 12:37 EDT-0400)