http://www.overclock.net/t/1508022/can-i7-2600k-utilize-2400-mhz-ram
Can i7 2600K utilize 2400 Mhz RAM???
Ok, it seems like it can if I overclock it, mine 2600K is set at 4.4Ghz. I am not sure if it really is able to use 2400Hz, but it seems it is fine up to ~2133 (?) . Though I wasn't able to find a clear answer about it.
And I still haven't got an answer for my original question.