Futureproofing my build

12GB DDR3 RAM, R9 280X, and a FX-6100 CPU.

 

I want to make sure my PC can handle next-gen games.  I haven't experienced much trouble so far, but I suspect that my CPU may be bottlenecking my performance.  What affordable CPU would you recommend to get me through the next couple years?

For gaming any mid-range CPU will keep you going. Most games are not too dependent on CPU performance. I cant speak for AMD but anything i5 equivalent should be fine. Especially if you can overclock. 

intel i5? k part so you can overclock.

I would wait until you actually need an increase in performance since prices will be cheaper and new CPUs will have come out as well. For a bump in performance in the meantime you can get a decent air cooler and overclock it if you haven't already. You can reuse the cooler on your next CPU as well if you make sure it's compatible.