Typically games are GPU dependent, especially the bigger ones like Battlefield, Crysis, Far Cry 3 and the other visually awesome games. A CPU overclock will give you some benefit, but dont expect much.
The games where it is CPU dependent is stuff like WoW, Skyrim, League of Legends/DOTA 2 and StarCraft II, where graphics arent that amazing and its designed to run on any system under the sun. Overclocking will give you big bonus here, but most of it will be "useless" performance. You'v raised your FPS from 55 to 75, that looks great on paper but in reality your monitor cant even display above 60FPS, and to do so will lead to screen tearing.
Overclocking your CPU is a nice performance boost, but in no way is it necessary for good performance (in games).