i5-2500k is a really good CPU. It's well rounded with amazing overclock capability. The i7-2600k is what you should go for if you plan to do 3D rendering and the likes. However, you should consider the $100 differences.
When it comes to gaming right now very few games will take advantage for more than 3 threads, because of this the value of hyper-threading on the Intel® Core™ i7-2600K is very minimal for games. That leaves the difference between the Intel Core i7-2600K and the Intel Core i5-2500K down to 100MHz for $100 that is why most of us advise that you go with the Intel Core i5-2500K and spend the money somewhere else that will make a difference like an SSD or something like that.