Ideally you want to get a CPU above 3.0 GHz for sure. All the work is done by the CPU - the graphics card is a bystander. Also the game cannot use more than one core.
My laptop at 2.6 GHz pulls 60 FPS right up until I start laying down firewalls (blending the graphics is quite CPU heavy).
My PC at 3.7 GHz is very happy at the cap of 80 right up until there's a huge group of players battling.
You should make sure there's not a lot of background tasks running - you can also set the game to run with a higher priority so that resources are not take away.