I just built a computer yesterday and my RAM readings are 1066mhz. The RAM I bought is PC-12,800-1600mhz.
If you look at the motherboard specs it says 2000 (OC) but not for 1600/1333/1066.
Is this just a matter of changing frequency or does this require overclocking?
AFAIK, for the RAM to run at 2000MHz, you need to overclock your CPU as well. If you want the RAM to run at 1600MHz w/o overclocking your CPU, then go into the BIOS, and in memory features or wherever it's located, and set these memory timings (if http://www.newegg.com/Product/Product.aspx?Item=N82E168... is your RAM):
CL - 8
tRCD - 8
tRP - 8
tRAS - 21
CR - 2T
Set voltage to 1.6V, lower is better, it's known as vDIMM or DRAM voltage.
Then afterwards set your FSB : DRAM ratio to 2:12 or if you don't have that option, set your memory multiplier to 12x instead.
What Lmeow said, but just as to why 1600mhz is considered an overclock, it's because JEDEC doesn't specify that high so there's no official standard for it. However, RAM manufacturers build their sticks to run at whatever speed they say - yours and mine at 1600mhz - but according to the mobo you need to "overclock" it because it is indeed beyond the certified JEDEC spec. If you have Everest, it will display the XMP settings, and generally if there's 2 profiles, the first is the manufacturer's certified setting, the 2nd is the extreme setting.
As above - You'll need to manually configure your memory to the settings on the package.
Having said that - Understand that faster RAM doesn't get you much in the way of extra performance (a percent or 2 at best), and that the additional overhead is intended to support faster bclk/fsb overclocking.