That cpu is absolutely a bottleneck. The benchmarks don't lie. It never makes the games unplayable but the bottleneck isn't exactly subtle if we're to believe the below benchmarks. Mind you, at 19201080, the gpu will still usually be the rate-limiting factor. The bulldozer based cpus are actually a lot slower than the phenom x4s, x6s, and the new Vishera based cards. OCing the cpu doesn't address the bottleneck all that well either.
AMD FX-8120 Gaming Performance | bit-tech.net http://www.guru3d.com/articles_pages...review,10.html AMD FX-8120, AMD FX-6100 and AMD FX-4100 CPUs Review. Page 3 - X-bit labs
Also, since minimum fps are usually significantly lower than avg framerates, the difference b/w avg. 40fps and avg. 70 fps will be noticeable.