I'd say that generally your CPU would be holding those cards back. Just depends on the game, settings, and resolution though. For just $200 though it's a great upgrade so I say go for it
. Overclocking your CPU will certainly help, but the original Phenoms don't go to far. You could drop a Phenom II 945 into your board (after a BIOS update) and overclock it. Not sure why your board doesn't support the 125W Phenom IIs.