upgrading to a higherend gpu like GTX970 and 290X the cpu is gonne make sense in allot of games. Since your motherboard isnt capable enough to overclock a FX 8 core, you need to upgrade your mobo anyway, if you want a cpu upgrade.
In this case, in my opinnion i would go with a 4690K simply because its just better in gaming. it has better single threaded performance on which 90% of the games still relay on. especialy on 1080p. a FX8 core cpu no matter how high you overclock it still falls behind even by a locked haswell i5 @3.4Ghz.
You also not gonne save any cash going with AMD, because if you wanne overclock a FX8 core, to 4.8GHz, to get anywhere near intel haswell i5 performance, you also gonne need an expensive mobo, and decent cooling. If you think about these extra costs, a locked i5 with a cheap H series chipset board for example would be cheaper, but will perform simply better in gaming. And them im not even speak about a 4690K.
In my opinnion its not worth it anymore to invest in a 3 year old platform. The single threaded performance of the FX cpu´s cant even keep up with Sandybridge.
But im not saying that AMD cpu´s are bad, because they are still very good, i have one my self. But i simply cannot deny the fact, that in in pure gaming, they simply show their age against haswell. If you do more then just gaming, like 3d rendering, or virtualization, for these kind of tasks the FX 8 core cpu´s are realy great. But for pure gaming a haswell i5 is simply the better allrounder now days.
Also about the guy that made a thread about not much performance gain, going from amd to intel. There was probably something wrong with his setup, because he claimed in a post on page 2, that he went back from intel to amd, and lost 30fps in ARMA III and 35fps in BF4 multiplayer, well i would call that an significaten loss in performance, or lets say an significant CPU bottleneck on the AMD side to his GTX970 Sli.
I personaly think, that AMD is basicly still fine if you go with a midrange GPU. and be happy happy with the performance you get out of a setup like that. But if you want great performance with a highend GPU R9-290 (x) / GTX970 / GTX980, then i would realy recommend intel at the moment.
I realy hope that DX12 api, is going to be a game changer for AMD cpu´s, That DX12 is gonne take allot of the overhead away from the cpu....
Fingers crossed.