PCPartPicker part list:
http://pcpartpicker.com/p/QXt9
Price breakdown by merchant: http://pcpartpicker.com/p/QXt9/by_merchant/
Benchmarks: http://pcpartpicker.com/p/QXt9/benchmarks/
CPU: AMD A10-5800K 3.8GHz Quad-Core Processor ($99.99 @ Microcenter)
Motherboard: Biostar Hi-Fi A85W ATX FM2 Motherboard ($74.99 @ Newegg)
Memory: Crucial Ballistix 4GB (2 x 2GB) DDR3-1600 Memory ($29.83 @ NCIX US)
Storage: Toshiba 500GB 3.5" 7200RPM Internal Hard Drive ($54.98 @ Outlet PC)
Case: Raidmax Atlas ATX-295WB ATX Mid Tower Case ($29.99 @ Newegg)
Power Supply: Corsair CX 430W 80 PLUS Bronze Certified ATX12V Power Supply ($39.99 @ Newegg)
Optical Drive: Asus BC-12B1ST/BLK/B/AS Blu-Ray Reader, DVD/CD Writer ($54.98 @ Outlet PC)
Total: $384.75
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-04-15 20:24 EDT-0400)
Under $400 and it has a blu-ray player...knock yourself out man. The onboard graphics will be good for medium on something like Diablo3, and you can always save $70-80 to buy a HD 7750 and crossfire it with the APU (later on).
If you were going to spend $400 on it, the one thing I would do, is go to 8 GB DDR3 1866 RAM if I was going to spend anymore money on that build.