Your video card is pretty serviceable, upgrading the platform would provide the best bang for your buck. AMD/ATi has been killing it lately in the bang for your buck category.
I was looking about after the IRC chat and I think this is the best CPU+Mobo combo you can get for $250:
ASUS M5A97 ($83.99, $73.99 with mail-in rebate) *Free Shipping*http://www.newegg.com/Product/Product.aspx?Item=N82E16813131872
AMD FX-8150 Zambezi 3.6GHz Eight-Core ($159.99) *Free Shipping*http://www.newegg.com/Product/Product.aspx?Item=N82E16819103960
It will work with your existing memory. 8 Cores @ 3.6GHz, overclockable (FX cpus are unlocked so you just change the multiplier) and built on a solid Asus motherboard. A quick Google search shows tons running at 4.5GHz on high end air and 5.0GHz on water. With a stock to decent air cooler I could see doing 4.0GHz easy. Its a great platform.
EDIT: As an aside once I have my new rig built I will be selling my current memory if you wanna good deal on a fast 16GB kit (I know the red memory and blue mobo will clash...):http://www.newegg.com/Product/Product.aspx?Item=N82E16820231499