Overclocking only helps for CPU intensive games like a 64 player lobby in BF3 or a Source game from Valve.
what is your cpu or cpu you want to get and what gpou you have or want to get.
i already have HD7970 ghz edition, and planning to buy 3770 or 3770k for OC if it will be useful in gaming ?
The i7-3770k is a waste of money for games. Yes, it is a very powerful CPU and you are paying a premium price for Hyper Threading which is a pretty good technology....
.... but games do not make use of Hyper Threading. You are better off buying the i5-3570k.
OC'ing can give a boost to games, but the exact increase performance depends on how much the game is dependent on the CPU. For example, I've seen benchmarks of Crysis 2 using various CPU and whatever graphics card. An i5-2500k running at around 4.8GHz got 1 extra frame than an i5-2500k running at stock speed. However, Skyrim is dependent on the CPU and GPU. OCing the CPU will give you better performance. How much I forget but it was something like for 1% CPU overclock you get like a 0.4% - 0.5% improvement in performance.