Since you said that you would like to stay with Radeon cards I have picked out a two that should work very well.
Higher End Card: AMD Radeon 6950 2GB GDDR5 Link: http://www.newegg.com/Product/Product.aspx?Item=N82E16814129171
Lower End Card, but still an awesome card: AMD Radeon 6850 1GB GDDR5 Link: http://www.newegg.com/Product/Product.aspx?Item=N82E16814102908
I would also HIGHLY RECCOMEND that you upgrade your Power Supply if you can swing it. Although it says it only needs 500W Power Supply I would upgrade just to be on the safe side and keep my system even more future proof. An 800W would do just fine.