Right now for games it would be very hard to beat the Intel® Core™ i5-2500K as the best processor for gaming. The issue is that very few games can take advantage of more then 4 threads and those that do yield very small improvements for additional cores/threads (you may get a % point or two).
Now in order for this to change something would have to happen in the compiler/programing front that makes it a great deal easier to add additional threads to the multi-threading support on the program. I have a friend that is a Ruby programer and he hates trying to program anything multi-threaded let allow trying get support for 4 or more threads out of it.
So while anything could change I don't think that the investment in the Intel Core i5-2500K could lead you wrong for gaming.
for gaming, the 2500k is the better choice. fx has more cores but most of the new games don't even properly use 4 cores. and by the time the games utilize 4+ cores, the 8120 wont be able to keep up with then-available processors.
also you get to upgrade to ivy bridge since they're backwards compatible with lga 1155.