DRAM can play in and provide increasing of 10% or more in FPS, but a lot depends on the individual game, i.e. Fallout 4 has shown good gains from faster DRAM. Also many don't notice, but the faster sticks tend to improve your average FPS also. Game developers are utilizing DRAM more and more so we can prob expect to see more increases as the newer games come out
With a 4790K, I'd look to 2133/2400 for the best bang for the buck, on Haswell, I lean towards Trident X, Ripjaws X and the Sniper lines of DRAM