GTA V (http://www.techspot.com/review/991-gta-5-pc-benchmarks/page6.html): FX chips are slightly behind Intel chips in performance, but are still more than sufficient to expect to hit at least 50FPS on Very High @ 1080p.
Witcher 3 (http://www.techspot.com/review/1006-the-witcher-3-benchmarks/page5.html): Pretty much just like GTA V. The gap is only going to be about 10% or so, which would translate to maybe losing ~5FPS (10FPS tops) on Ultra @ 1080p.
Fallout 4 (http://www.techspot.com/review/1089-fallout-4-benchmarks/page5.html): FX chips will give you about 50-60% of the Intel performance. However, with a GTX 1060 that should still be enough to give you at least 40-45FPS on Ultra @ 1080p, so turning the quality down a level should bring you up to at least 60FPS.
Overclocking is going to be hit-or-miss. In some games, overclocking scales quite well; in other games, there's very little change (whether it's an AMD or Intel chip). Fallout 4 seems to scale fairly well, so if you can push the FX-8300 up to 4GHz you should expect to see ~15-20% better performance, getting it to 4.3GHz should give you ~25-30% more performance.