If you're up for some reading I can point you to some articles that are informative and should help you answer that question.
Most of the articles agree the differences between each step up in RAM is often marginally noticeable, and outside of a benchmark it is difficult to say whether or not you would even notice the difference. And the cost difference can add up fast. But there is a difference and a decision has to be made when putting a computer together, what's best for you as the builder and user of the pc. The best advice I got from these forums was buy the best RAM that fits your budget, if you find yourself stretching things just to get 'better' RAM that's probably money better spent on other components.
Both of those articles were written about the i7 9xx / 1366 socket chips. I haven't kept track of any articles testing the difference on AMD though I could probably dig one up. I also haven't seen a lot of articles specifically focused on RAM speeds on the P55 platform. However, Tom's has had two.
First up: http://www.tomshardware.com/reviews/core-i7-870-1156,24...
I didn't care for the way results are presented, to me it's not as clear cut as it could have been. I think it points out there's little difference between marginally slower and faster RAM but the way the results are shown leave room for individual interpretation. Also the test page (and the URL) states that they used an i7 870, but in the opening they (still!) say testing was done with an i7 860 . Still worth a read.