So far as I can tell, there is no such thing as an FX-6800.
GTA V runs very poorly on AMD FX CPUs, since it requires very fast single cores. You can probably expect 25-40fps averages in most places, due to CPU limitations.
Well actually it's not going to be that bad, I currently have fx6300 paired with a gtx760 and I see no bottleneck from the cpu. I average about 50fps on modified settings which are close to being very high.