http://www.cpu-world.com/Compare/571/Intel_Core_i5_i5-2500K_vs_Intel_Core_i5_i5-4670K.htmlalthough the newer one is faster by around 10%, the old one is certainly not bottlenecking your 670 (or whatever you have) so even after an upgrade you won't see too much of a difference, if you're lucky just 5 FPS. Not really worth a 200 dollar upgrade in my opinion.
The upgrade would be much less than $200. I can sell both. Also, I'm running two 670's in SLI. Don't know if those together could be bottlenecked.