if your CPU bottlenecks your GPU, your frames will drop a bit, like maybe 5-20 fps, which will vary in different games, and when you say its 15fps, does it run constantly at 15 fps?? does this happen in other games such as BF3, Starcraft2?? and moreover the games which you mentioned are bound to be CPU intensive as they tend to load large maps, but then you shouldn't get that much of a fps drop.. im using a 670 with i5 2500k oc'ed and my fps drops sometimes to 30-ish on GW2 when switching maps but then it goes back to normal after a second or two.. and as the others suggested, id swap the cpu and mobo for an intel setup.. also how high is your CPU and GPU usage in the said games, if GPU usage is low, it means that something is bottle-necking your GPU (prolly the CPU), you can check the usage with MSI afterburner, and if your CPU usage is unusually high even when the system is idle, some background application is slowing it down.
here's a list of things you can do:
1. complete uninstall of video drivers and reload the latest drivers - 314.22 nvidia drivers
2. Run disk defrag
3. Scan the PC for virus
4. Check CPU temps and apply thermal paste if necessary - could be CPU overheating
5. Check your temps on GPU, it will throttle down when it reaches 70degs (this doesn't actually affect FPS that much)
6. Check CPU load and memory usage while gaming and on idle to see if any background processes are slowing it down, if your CPU usage is 100% while gaming, you are cpu bottle-necked
7. Full re-install of windows
If after doing all the above, your fps still drops.. id suggest upgrading the CPU+ Mobo combo to an intel setup, and do not get the bulldozer, its a good CPU that i know, but in terms of gaming, you will get better performance out of a core i5, so get the best intel cpu+mobo combo you can afford