The i7 960 is a quadcore with hyperthreading - meaning it can process eight logical threads simultaneously. This would help a lot if you're doing video editing (or with any heavily-threaded applications), but otherwise does nothing. The 900 series also uses triple-channel memory (for Socket 1366) so you have to buy three sticks of RAM to fully unleash its performance.
The i5 2500k and i7 2600k are both quadcore, but the i5 does not have hyperthreading. Again, this only makes a difference for heavily-threaded programs. If your computer is for gaming, you'll never notice a difference. Also, the ones ending in K can be overclocked very easily (since the multiplier is unlocked). The i5 2500k is only about $200-220 online, and the extra money you saved over buying an i7 960 can be used to get a faster graphics card and more RAM.
If you're balls-to-the-wall for more video editing performance, get the i7 2600k. Otherwise, stick to the i5 2500k.