The Tech Report did a mini-study/real-review of the impact of memory speed on Core i7 performance, to specifically answer your question.
The difference between i5 and i7 aren't great in this respect, so you can draw conclusions from this article.
Please, please, please ignore the synthetic benchmarks (WorldBench, Memory performance, etc.) and just look at what you're interested in: the gaming section.
The results are sobering. Ignoring the different processors being compared and just looking at one processor vs the other at a different memory speed, the difference amounts to about a 3% increase in performance with higher speed ram, e.g., about 2fps on a 60fps game or what would amount to about 1fps on a 30fps game (fps in this instance would mean what your gaming rig could produce initially, not that a game is set to that fps!).
If you're looking to spend a bit more to get more speed, use that money on a better graphics card.