Intel Core i5-2400 Sandy Bridge 3.1GHz (3.4GHz Turbo Boost) $190 Is a downgraded version of i5 2500k
Intel Core i5-2500 Sandy Bridge 3.3GHz (3.7GHz Turbo Boost) $210 i5 2500 is mid range CPU for gaming and only allows minor overclocks
Intel Core i5-2500K Sandy Bridge 3.3GHz (3.7GHz Turbo Boost) $220 CPU is unlocked, which can allow you to get 4.5Ghz easily
And the Intel Core i7-2600K Sandy Bridge 3.4GHz (3.8GHz Turbo Boost) is $300 dollars. Is it worth the upgrade from i5 to i7? Only differences between this and i5 2500k is that i7 supports hyper-threading. Which will excel in rendering and heavy tasking
But have you try looking into Ivy Bridge CPU? They are top notch performer.
Hope I helped.