Which processor best buy?
intel i7 2600 or intel i7 2600k.
What are the differences besides the price?

2600 and 2600K in exactly the same thing. The main difference between these two is that the K series are unlocked, as someone said earlier.

What is unlocked mean? This means that you can overclock very much, I have read several articles about a guy who can disperse up to 7 GHz!
But this is to go to extremes. Normal people with apropiate (and inexpensive) cooling system can overclockit to 6 GHz (with Turbo).

In the other hand, 2600 is "limited unlock." It basically means that you can overclock a bit, up to 4.2 GHz (with Turbo).

So, if you freak OC, or plan to do OC, $ 25 more for the unlocked version is very good value.

Tom100, If you are not going to overclock you processor, you better buy a simple version.
Because overclocking is not so simple thing. You should balance cooling, power systems for that.
Keep in mind, that any overclocking is risky.

p.s. But i will buy i5 2500k. Optimal for me.