What modifications (if any) should I make to this design

You'll get better gaming performance out of the following build...

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: AMD Phenom II X4 965 Black 3.4GHz Quad-Core Processor ($84.99 @ Amazon)
Motherboard: ASRock 970 PRO3 ATX AM3+ Motherboard ($70.56 @ Newegg)
Memory: Patriot Intel Extreme Master, Limited Ed 8GB (2 x 4GB) DDR3-1600 Memory ($31.99 @ Amazon)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ NCIX US)
Video Card: Asus Radeon HD 7770 1GB Video Card ($89.99 @ NCIX US)
Case: NZXT Source 210 (White) ATX Mid Tower Case ($35.98 @ Newegg)
Power Supply: Corsair Builder 430W 80 PLUS Bronze Certified ATX12V Power Supply ($19.99 @ Newegg)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer ($17.89 @ Outlet PC)
Monitor: Acer G236HLBbd 60Hz 23.0" Monitor ($109.99 @ Newegg)
Total: $521.37
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2012-12-25 20:50 EST-0500)

There's no need to get the FX 6100. The phenom x4 965 will run circles around it in games. You also don't need an aftermarket cooler right now. The stock cooler that comes with the CPU will work fine.

As for the GPU, I would recommend you change it to the HD 7770. It'll get you far better performance than the GPU you chose. I also switched out your monitor for something far better for not much more than what you had originally chose.