The i7 3770K has Hyperthreading so is 4 cores with 8 threads. Hyperthreading is not useful for gaming but it better for video editing and rendering.
The i5 3750K is a quad core with no Hyperthreading so is just 4 cores 4 threads. It is better for just gaming.
Since you are looking at "K" model processors I assume you will overclock? Ivy Bridge is very good and roughly 8-10% faster clock for clock than Sandy Bridge but it gets hot, as such Sandy Bridge chips will probably overclock higher thus overcoming the slight speed advantage of the Ivy Bridge CPUs. An i5 2500K is still about as good as you can do for a gaming CPU.