For those who do not already know - the RAM speed advertised is the TESTED frequency that the RAM has demonstrated it can run at stable on a test bed. This is typically NOT the default speed that the RAM will boot at unless it's low speed DDR3 type RAM like 800/1066 RAM.
In order to run the ram at a higher speed than your CPU's default RAM speed - which is what the RAM usually boots at, you need to go into the BIOS and manually set the RAM frequency, latency and sometimes voltage if it requires more than the default DDR3 1.5V. On Intel CPUs you can select the predefined XMP profile if your mobo BIOS offers this option.
Understand however that just because the RAM is capable of running higher than the default CPU defined RAM frequency does not mean the CPU based memory controller will also run OC'ed at as high as the RAM will run.
Overclocking is a journey of exploration where you try to see how fast your particular hardware components will run stable. Your hardware may be the same as other folks but it can perform completely different than other people's, so you can only use their OC'ing results as a reference point - not a guarantee of what your hardware will do.
As an example you can use DDR3 RAM tested at 2133 Mhz. with AMD Phenom II CPUs. The default RAM speed is 1333 Mhz. Most Deneb chips will run the RAM stable OC'ed to 1600 Mhz. and not much more. The Thurban X6 Phenom II chips also have a default 1333 Mhz. RAM speed but some folks are able to run the RAM at 2000 Mhz. but not much more. So if you buy 2133 Mhz. RAM it does not mean that your CPU can run the RAM at it's full frequency. You get what you get.
FWIW there is little system gain just from OC'ing DDR3 RAM so you're not missing out on much if your CPU won't OC the RAM a bunch.