I've had this rig for a while, I've noticed that on the games I play, like skyrim and Guild Wars 2, that I get lower frames in what I think is more CPU heavy places. Like in Skyrim: Whiterun. Whiterun I always get low FPS in plaves where there is smoke coming from the torches, but I also have to put in to consideration that my GPU has PhysX. I also have to turn water reflections off in GW2 because it slows me down drastically.
I was thinking of upgrading my system to an intel motherboard and getting the i5 3570k.
Honestly, as crap as the FX cpu's are, I don't think its bottlenecking. It just isn't able to perform as good as for example a 2600k, but its not bottlenecking, it might also be your ram or GPU, but its always a good idea to upgrade to intel if you have the money.