Get an HD 5670 because it doesn't require much from the power supply and its up to date with modern games/technology.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161334&cm_re=HD_5670-_-14-161-334-_-Product
Thats what would be good for your system, so long as you have a normal PC tower. Pretty much any PC can take one of these.