The speed of the ram does not matter , you can put 32gb in there if you want to. The question is why would you , do you have some applications that use a tremendous amount of ram? You will not even use half of it for gaming , any game. If you want to get the best use of that speed and all you use the Pc for is gaming then get 8gb of the 2133mhz ram , if you want to go higher then get 16gb. I have heard that the computer uses the ram more efficiently if all the slots are not populated and the max amount is not installed.
Gaming takes absolutely no advantage from higher performance RAM. Higher performance RAM for gaming is a gimmick to get gamers to waste money without providing a better experience. High performance RAM only helps a few applications such as compression.