Do I need Core i7 Core i5?

most games will only use 4 cores so for gaming go for the i5 but the i7 would be better for video editing.

Depends on budget and priority really.

You are in a different situation to many posters whose only CPU intensive use is gaming. For those people, i5 2500K is the better choice (not because it's better, but because it's cheaper and no worse than the i7).

Because you want to edit video, the i7 is unquestionably superior. Larger cache and hyperthreading yield perhaps 15-20% better performance. I also edit video and for me, this performance advantage is decisive. But what about you? Is this much of a performance increase worth the extra cost? Would saving money on the CPU allow you to get a better GPU and is the better performance you'll get in games worth the lower performance in video editing? This is where it comes down to your personal preference.