This pretty much depends on the data set of data you are working on. To put it in to perspective, I ran my machine at DDR3-1600 and ran a certain loop around the world in Rift, and then switched up to DDR3-2133, and I went from about 70FPS -> 72FPS average. The problem is, because it's an MMO, that could be because two people weren't standing in a specific spot so my overall average went up.
There was no noticeable difference in game play in any game I play. Though, I didn't do many tests on other things, because anything else I run on my machine is so quick there isn't much point in tweaking for it.