As stated for Gaming, very little diff as game performance is MUCH more closely tied to the GPU.
Number of cores (over 4) only becomes an issue if you use specialized software - that can also be expensive. Performance per core, no contest - I5-2x00k.
It is not always about the CPU. You also have to pare it with a Motherboard and the chipsets used. Amd has been a little on the slow side in getting out optimized drivers for their chipsets Case in point is how long (if they have) a driver for SSDs that match Intel's iaSTor driver. Also AMD has not provided a good driver set for swithiching between ISP and a dedicated GPU when switching from 2D to 3D applications. 3rd party developers, be it hardware or software; If they have to choice which systen to optimize it for - It will be the intel system, pure and simple AMD only around 20% of market share, Intel over 70 %. In most cases they design for the intel and let the chip falls for AMD. Not really an INTEL fanboy, this just a fact of life.
If gaming go with the i5-2500k, if use specialized software and need the extra cores, the 2600k and for iether pare it with a Z68 MB.