If you are in the US one of these;
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121376
http://www.zipzoomfly.com/jsp/ProductDetail.jsp?ProductCode=10012196
Assuming your power supply can handle one.
This card isn't quite as good as those but is a very good deal for the money after the rebate;
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127495