I wouldn't really go above an HD 7850 or HD 7870 with an FX 4100. If you had an FX 8XXX CPU, it would make more sense, but putting the lowest end FX CPU with a rather high-end GPU doesn't sound like a smart move. OC the CPU if you haven't, just so the bottleneck is lessened if you go the HD 7850/HD 7870 route.
Sorry I couldn't answer earlier. Actually, looking at it from a future proofing and upgrading perspective,I would recommend the HD 7950 at this point.
With the HD 7950, you'll (obviously) see higher performance with it than the HD 7870. Of course, a bottleneck will be present and more significant than with the HD 7870 or HD 7850, but your OC should help compensate for the bottleneck (won't remove it though). I assume you have a decent AM3+ motherboard, so you can always upgrade to a better FX CPU down the line and keep the HD 7950.
Basically, just get the best you can afford at this point, and eventually upgrade the CPU so your card isn't as held back with the FX 4100.