Is it 1080p or 720p?
Well since we now know your specs i'm going to suggest going with a 5770/6770.Anything higher than that will get bottlenecked by your CPU.Of course you can always O.C. to lessen the bottleneck.
Your also going to need to upgrade your PSU.Here's what I suggest.
MSI Twin Frozr II 6770 $140=$125 after MIR + $8 Shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16814127590
Antec Gamer Series 520 watt $60 + Free Shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16817371047
Your total should be around $200 before rebates.If your upgrading from the intergreated graphics it should yield a huge boost in performance.