As you didn't get it yet, my opinion is that you really should rather build one yourself. It is really, really easy to do, you can learn everything about it by watching some Youtube videos for 20-30 minutes. This way, you can get a much better build all around, and you won't need to add a new GPU on top of it. Here's what my recommended build would be :
PCPartPicker part list:
http://pcpartpicker.com/p/1BPr7
Price breakdown by merchant: http://pcpartpicker.com/p/1BPr7/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1BPr7/benchmarks/
CPU: AMD FX-6300 3.5GHz 6-Core Processor ($117.98 @ Outlet PC)
Motherboard: MSI 970A-G46 ATX AM3+ Motherboard ($79.98 @ Outlet PC)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($59.98 @ NCIX US)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($59.98 @ Outlet PC)
Video Card: Sapphire Radeon HD 7850 1GB Video Card ($129.99 @ Newegg)
Case: Corsair 300R ATX Mid Tower Case ($49.99 @ Newegg)
Power Supply: XFX 550W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($44.99 @ NCIX US)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Newegg)
Operating System: Microsoft Windows 8 (OEM) (64-bit) ($88.88 @ Outlet PC)
Total: $646.76
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-09-09 14:14 EDT-0400)