i5 4670k and FX 8350 are almost a dead on match in most games, with the 8350 pulling ahead slightly in newer, heavily threaded games. But not by much.
They are also very close on cost, with the i5 paying for itself in 2-3 years of use from power savings.
Both mainstream and high end motherboards for AMD are slightly less expensive than comparable Intel boards. But again, this is fairly close.
All things considered, they are so close at stock speeds and with good overclocking, that it really doesn't matter much for just gaming.
* Caveats *
Areas where an 8350 or i7 4770/4790k will be better than the i5 4670 are streaming, video editing, and some heavily threaded games and apps.
*IF* you decide to get the i5, go with the new Devils Canyon version. Pretty much tha same price, better TIM on the IHS, and slightly higher stock clock speed.