The Fx series is not AMD's best creation, however it should be ok with the hd7850. Whats your budget? You could get a nice i5 that performs better.
Also what are the other components that you are choosing?
I would get a phenom II x6 if you can, as it is better than the fx6100. And the i5-2500k as an example is a much better choice. You should not see any major bottleneck, but what are your other options?
There might be a small bottleneck at times, as you saw in the review links i sent you for the fx6100, if you overclock it will perform better. But the fx8150 is not a perfect processor either. So if you plan on getting a fx8150 you might as well for that price get a i5 2500k.
Well the 8150 is the best of the bunch, but the 6100 or the 8120 will be your best bet, if you can't get the top dog. The 8120 is better than the 6100, as it has 8 cores rather than 6, and gets higher frame rates.