http://www.newegg.com/Product/Product.aspx?Item=N82E16814102999
The HD 7850 is a pretty solid card I use on my 2nd machine which has a similar CPU to yours. You could probably get away with keeping the Athlon II but if you wanted to upgrade you could probably slot in this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103727
Which would cost you $85 after the rebate card and it would put your overall bill at less than $300.
Alternatively you could go with something from an Intel flavour if you wanted to pay a little more and get a new motherboard etc.
Tease.