You could settle for a HD 7850, packs more of a punch than the HD 7770. I myself own the HD 7770 and its quite a champ for gaming at 1600x900 but im guessing your gaming at a higher resolution so you'd want something more powerful.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161426&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=