Personally, I would go for the i5 2500K (Btw why the K? You planning to overclock?)
I read somewhere windows isn't even capable of fully using 4 cores to it's full extent, let alone 8 of 'em. Plus, it's not like you will use 8 cores for gaming, must games only make use of 2 of 'em. If you plan to go video/photo editing or some other heavy duty programs, then you might want to consider the 8 core but meh, I'd get the i5.
If you're unsure which i5 to get, I'd pay $50 more and get the 2500 one. This way there's no way you'll ever have any regrets about it and 4 x 0.30 Ghz more is kinda something lawl =3.
Hope I somewhat helped, if anybody asks. I do not have any sources about the windows-cant-use-all-cores thing, thought I'd read it somewhere and I'd be happy if anybody could tell me wrong ^^,