The i7 is not overkill, but is largely useless for gaming.
Few games can use more than 2-3 cores, so the extra hyperthreads of a i7 are not very useful. The price premium of a i7 is better used on the graphics card budget.
The FPS you get with a i5-4670K and a mild overclock will be determined by your graphics card and your settings.
A GTX780 is entirely appropriate for a 4670K.