For the 2500K, anything beyond DDR3-1600 and even DDR3-1866 is just a tiny, marginal benefit not even worth anything except maybe a slight difference "on paper". Beyond DDR3-1600/1866, it just becomes extra e-peen to be honest unless you are aiming for the highest benchmarks so that you can win competitions.
This article was extremely helpful when I was buying memory for my 2500K (see my sig rig for the memory that I went with and am very happy with): http://www.bit-tech.net/hardware/memory/2011/01/11/the-best-memory-for-sandy-bridge/1
Study it, and I promise you that you should be able to make a confident and informed decision.
(Now of course, this isn't so true for newer CPUs; that sweet spot goes higher, but I suppose that's expected)