I'm a bit of a novice but I believe I can answer this one.
RAM will try to run at whatever speed your mobo teels it to run at. Certain ram is already spec'd and marketed to run at certain speeds (ie. 1600MHz, 1866MHz, etc) by the manufacturer.. but, any stick of RAM can be overclocked or underclocked. The question is... is it good enough to actually work lol.
I've heard from people who've OC'ed their 1600MHz vengeance RAM to 2133MHz. No clue how they did it..... I can only get my Kingston HyperX Red 1600MHz to run at about 1933MHz.