1600 vs 1866, you will not see any noticeable performance differences. Only a minimal difference in benchmark software results. I always say get the 1600mhz ram as it would be more stable. Until you can run 1866 or higher ram without overclocking it, stay with the boards max speed without an overclock.
I hopped on Dell Technical support chat and the guy pretty much said the same thing.
He was really helpful! - He indicated that over clocking RAM is not supported by warranty but gave me some options.
So I stayed on the safe side as there would be no noticeable difference - except maybe system instability.