The 4790k will be a whole new world compared to a FX6300. But big fps increases in gaming, come from the GPU, not the CPU.
I'd still upgrade the CPU first, because if you get like a GTX970 now, it will be bottlenecked by your FX6300.
I'd also just get the 4790k if you also do editing or have the money to blow. If you're just gaming, an i5-4690k/i5-6600k will do just as good as the i7.