depends on the game. some, like Starcraft II will see a significant boost, while others, like BF3/4 SP & usually 3rd person action games will see as little a difference as an upgrade from an i5 to an i7 for gaming (*hint* *hint*)
EDIT: right, the GPU; most people usually use MSI Afterburner to OC their GPU. you basically raise the clock rate a tiny bit, test it's stability & temps with something like Furmark, repeat the 2 steps till you either experience a problem (BSOD) or high temps.
for the former, you turn it down back to the last stable settings and either keep it that way or, provided your 780ti isn't voltage locked (do take note), raise the voltage a bit & keep on OCing till you either face another stability issue and/or high temps. you'll eventually have to stop for either one.
I know I should get an i5 for gaming. I do a TON of video encoding to convert to DVD and burn DVDs so that i7 will help there too. Truly this is a present for my graduating in college to myself. 2000 beast I've always wanted, coming from a laptop
I don't think there will be any real difference. In most (MOST) cases, the processor is not a bottleneck, and performance is directly limited by the GPU. That is especially tru when talking about such a high end CPU, so you will not notice any difference.