Welcome to tsf. You would idealy want another ati card an not nvidia as the drivers issues between the two are a hassle. With only 365watts a 2600 pro (not xt) with ddr2 only is as high as I would go.
http://www.newegg.com/Product/Produc...82E16814131115
http://www.newegg.com/Product/Produc...82E16814161098
The Powercolor is the cheapest but the HIS has a very good cooler.