Ok so you need to upgrade your power supply and then you can get a better graphics card.
In theory this should be fine:
PCPartPicker part list:
http://pcpartpicker.com/p/2kZZV
Price breakdown by merchant: http://pcpartpicker.com/p/2kZZV/by_merchant/
Benchmarks: http://pcpartpicker.com/p/2kZZV/benchmarks/
Video Card: PowerColor Radeon HD 7850 2GB Video Card ($134.99 @ Newegg)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($54.99 @ NCIX US)
Total: $189.98
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-12-15 21:44 EST-0500)
But if your case can take it or not, is difficult to say.