``At first it would appear that the answer to the question was going to be easy as the first 3 tests (3DMark 11, UNiGiNE Heaven & Metro 2033) surprisingly showed no real variation in performance as we modified the CPU speed. This was even more surprising as thanks to the extreme overclock ability of the Intel Core-i5 2500K I was able to vary the CPU core speed by over 1.3 GHz. At this point I would be suggesting that you run your processor at close to stock speeds (perish the thought!). But then along came the Crysis benchmark and at last we saw some more interesting numbers beginning to appear. As you can see we gain a good deal of FPS between running at 3.5 GHz and 4.1 GHz, roughly 7%. Pushing the CPU further all the way to 4.8 GHz only yields a further increase of around 2%. Suggesting that for Crysis the optimum CPU speed would be around 4.3 GHz. Finally we have Just Cause 2 showing similar results in the fact that during the most GPU intensive benchmark (Concrete Jungle) we gain around 8 % between 3.5 GHz and 4.8 GHz.`
http://www.pcgameware.co.uk/articles/how-fast-does-my-cpu-need-to-be/
http://www.pcgameware.co.uk/articles/how-fast-does-my-cpu-need-to-be/