Most games can use only 2-3 cores. They depend on the graphics configuration more.
There are some exceptions like FSX or BF3 multiplayer which are cpu dependent.
But, by and large an overclocked quad w/o hyperthreading(i5) is about as good as it gets for gaming.
I might suggest a 4670K and any Z87 based motherboard would be appropriate for a GTX780.
Definitely another good option.