For a gaming PC, the I5 would be the way to go. It will provide better all around performance in nearly all applications.
That said, an overclock on a FX6300 is not guaranteed. All of the FX6300/6350 I have had (10+) have been able to hit 5+ Ghz on air or water with a decent motherboard (Asus M5A97 or Sabertooth 990FX). Even at those speeds in applications that can use the 6 cores, the FX6300 isn't much faster if any, than a Haswell I5 at stock speeds.
Also, remember that the FX6300 with a decent overclock will use nearly twice the power of the I5 and on single or lightly threaded apps, the I5 will still be noticeably faster.