The opteron is exactly the same as the X2's as far as gaming is concerned.
The Opteron stock air cooler is fantastic with the heatpipes etc, you can reach 2.6GHZ on the stock cooler, depending on your ambience temps (the temps surrounding your PC case, and in your case)
You really should get the Opteron. It is better than the X2 because of the OC ability and Stability. Remember these are Server Chips so they can take the huge loads.