I'd switch to a Phenom II X4 processor. $20 gets you an extra physical core, a higher clock speed, and the potential to overclock in the future.
There's a little to be saved on memory by downgrading to some Mushkin 1,333 MHz, and a lot on that SSD that's gobbling up your budget. For the small difference it might make in loading times and stuff, I'd take a Seagate 1TB instead.
The graphics card is weak for the money. For $20 more there's a hugely superior HD 7870. It should be able to whack out roughly double the FPS which the 550 Ti is capable of.
Just look at the comparison, click here. Plus you get Crysis 3 free! (Sell it if you don't want to play it.) It's a no-brainer, this is the card you want.
I'd go for a different case, such as the NZXT Source 210, which chops $25 off, and is better quality.
Did you forget a power supply? The Corsair CX430 will do the trick for now.
At the end of all that, there's just about room to squeeze a considerably better motherboard, thanks to the cutbacks we made elsewhere.
PCPartPicker part list:
http://pcpartpicker.com/p/1c6Dq
Price breakdown by merchant: http://pcpartpicker.com/p/1c6Dq/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1c6Dq/benchmarks/
CPU: AMD Phenom II X4 965 Black 3.4GHz Quad-Core Processor ($84.98 @ Amazon)
Motherboard: Asus M5A97 R2.0 ATX AM3+ Motherboard ($84.99 @ Newegg)
Memory: Mushkin Essentials 4GB (2 x 2GB) DDR3-1333 Memory ($32.63 @ Amazon)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($64.98 @ Outlet PC)
Video Card: MSI Radeon HD 7870 GHz Edition 2GB Video Card ($169.99 @ Newegg)
Case: NZXT Source 210 (Black) ATX Mid Tower Case ($34.99 @ Amazon)
Power Supply: Corsair Builder 430W 80 PLUS Bronze Certified ATX12V Power Supply ($24.99 @ Newegg)
Optical Drive: LG GH24NS95 DVD/CD Writer ($15.98 @ Outlet PC)
Total: $513.53
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-07-01 05:07 EDT-0400)
It's about $20 more than the original, but much stronger. Hope this helps!