Nice system you have there! It already kicks enough ass, so the upgrade I'll suggest is quite tame. Sure, you could upgrade your motherboard or cooler and get a little bit more out of your CPU - but that's all - a little bit.
If you're after more gaming performance, upgrade your GPU. Sell your current GPU, which would get a couple of hundred dollars and yourself two new fast single-GPUs. Two Sapphire Radeon HD 7950s, or two new GeForce GTX 670s.
You'd see a near doubling in gaming frame rates, easily. I wouldn't bother upgrading your CPU until you're completely bottlenecked by your GPUs. You're already running a Core i7 at 4.5GHz - that's more than enough. Cranking up to 4.8-5GHz won't give you any gains at all in games from 4.5GHz.
Third-generation Core processors are of course better, and they will give you slightly better performance in everything (including games), but the investment would be better injected into your GPUs - in my opinion. If you were running a stock 2600K, then I'd suggest overclocking or maybe an upgrade - but you're already sitting on 4.5GHz.
Get some new GPUs! :)