The 2500K processor only supports DDR3 memory speeds of 1066 and 1333 any thing higher than that will require an overclock. That can be done manually in the EFI or more imply by using XMP. The problem with either of these, however, is that doing so will disable the turbo boost and idling features of the i5 by forcing it to accommodate the higher memory speeds. It can make sense to get higher speed ram, underclock it for every day use thus getting the benefits of turbo boost, and overclock it when I deem it necessary.
That's what I gather so far. The last thing I'd like to know is if I can get a memory module like this one and underclock it, but get lower CAS timing.