It's not Battlefield 3, but it's pretty solid nonetheless. I just started playing and there's plenty of populated servers at any hour. My ISP is having issues and my ping is usually in the 150's at best (at the moment, technician is coming out to take a look next week) and yet the game is...
You probably shouldn't consider AMD, since your criteria is a high powered upgrade from an e6700 @3.6Ghz. Go for the QX6850, that should net you 3.6Ghz with ease, matching your current setup but with the obvious advantage of quad cores.
AMD's quad core offerings will almost certainly be more...
The first chip I overclocked was a 3.2Ghz P4 Prescott. I got it to 3.8Ghz until settling back on 3.2Ghz but with uber undervolting to reduce the "space heater effect".
What kind of difference have you noticed with the new PC? I have a similar comp to your old setup and have been considering an upgrade to C2Duo for a long while now.