Overclockers Club published a review on the Does Memory Speed Affect Gaming Performance; An Intel Question A quote from the article:

After running through all of the tests, it appears that what I previously thought was an easy and clear answer is in fact more complicated. With the evidence provided I can safely say that memory can play a large role in some games over all frame rates. However, other factors like the processor, type of video card, and resolution will usually provide bigger impact in the final frame rates. Strictly speaking of game performances, the fastest memory tested does yield better results. Some games average FPS was within a 3% margin of error and others were wildly different. I think what any reader should take away from this article is that they need to factor in the rest of the system and make a sensible decision based on their setup. If you have money to burn or like to go full throttle, then you are probably already planning an SLI or Crossfire setup. At that point you might as well spend that little bit of extra memory to receive a faster set of memory. However, sometimes the jump in prices from one set to another can be drastic and that money could be better spent across the system as a whole.

 Does Memory Speed Affect Gaming Performance; An Intel Question @ OCC