AMD does not make any good microATX motherboards in the AM3+ platform. If you insist on going with AMD, this is your best bet.
PCPartPicker part list:
http://pcpartpicker.com/p/3JYvJ
Price breakdown by merchant: http://pcpartpicker.com/p/3JYvJ/by_merchant/
Benchmarks: http://pcpartpicker.com/p/3JYvJ/benchmarks/
CPU: AMD Athlon X4 760K 3.8GHz Quad-Core Processor ($84.73 @ OutletPC)
Motherboard: Gigabyte GA-F2A88XM-D3H Micro ATX FM2+ Motherboard ($64.99 @ Amazon)
Memory: Patriot Viper 3 4GB (1 x 4GB) DDR3-1600 Memory ($35.99 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($54.43 @ OutletPC)
Video Card: PowerColor Radeon R9 270X 2GB TurboDuo Video Card ($180.00 @ Newegg)
Case: Fractal Design Core 1000 USB 3.0 MicroATX Mid Tower Case ($39.99 @ Micro Center)
Power Supply: EVGA 500W 80+ Bronze Certified ATX Power Supply ($34.99 @ NCIX US)
Total: $495.12
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-05-14 14:44 EDT-0400)