The only good AMD FX CPU is the FX-81xx CPUs and that is only very 8 or more threaded apps, and even then, it is not SUPER great.
The Core i5 2500k really holds its spot in the gaming realm, and the 2600k is slightly better yet. The difference between the 2500k and 2600k are that the 2600k has hyperthreading (4 cores 8 threads) and 2mb extra L3 cache, which isn't that important to gaming since the 2500k has enough. The 2500k has the best performance/price ratio and is basically as good as the 2600k in gaming because most games don't use more than 2 cores, and barely any use more than 4.
If you have money to just blow, get the 2600k, but if you are on a budget, get the 2500k and a better GPU. Also, both the 2500k and 2600k will overclock VERY well!
for gaming, core i5 2500k is the best among the ones you listed.
fx cpus bottleneck cfx/sli, huge power hog, takes higher overclock to reach counterpart cpu's at-stock performance.
2600k is too costly, doesn't offer significant performance improvement right now, the money saved by buying a 2500k can be spent on a better gfx card or ssd.
for gaming, gfx card > cpu.
tom's gaming cpu hierarchy chart. http://www.tomshardware.com/reviews/gaming-cpu-overcloc...