Your aging AM2 platform is really holding you back. Almost any card you get that will be faster than the 8800 GT will be bottle-necked by your old Athlon 64 X2 6000+ dual core. And there really isn't any CPU you can replace it with that will make much of a difference. I suppose you could throw good money after bad and go for something like a GTX 1050 Ti or GTX 1060 3GB and see what that will provide on games that aren't terribly CPU demanding.