The graphics card will really determine gaming performance if you can really get each CPU to 3.2GHz. e2160 should be pretty straightforward to at least 3GHz; similar OCing of the Opteron will be much harder and will run much hotter.
I think getting a better video card would prove more fruitful. Not to say that a faster CPU wouldn't help. But, you are already running dual cores at 2.56 ghz, so the jump wont make a huge difference with a 2600xt.