Perhaps this explanation will help.
The following information came from this
RAM speeds can be quite confusing, as they can be expressed in several ways. Starting with the oldest DDR modules, the basic models run at an internal frequency of 100MHz, while more advanced modules increase the internal clock speed to 133MHz, 166MHz and up to 200MHz.
It might seem logical to refer to these different modules by their internal speeds but, thanks to the double data rate that gives DDR its name, a 100MHz module can carry out a theoretical maximum of 200 million transfers per second, while the 200MHz module can carry out 400 million transfers per second. For this reason, 100MHz DDR is known as DDR-200, 133MHz modules are labelled DDR-266 and so forth.
This is a fairly obvious system, but RAM transfers aren't very convenient units to work in. It's much more common to talk about data in terms of bytes. So to make DIMM speeds more easily understandable, they're also given a "PC-rating", which expresses their bandwidth in megabytes per second.
PC ratings can be calculated very simply. Each RAM transfer consists of a 64-bit word, or eight bytes. So to convert transfers-per-second into bytes-per-second, you simply multiply by eight. DDR-200 is thus equivalent to PC-1600.
DDR2 uses almost the same naming conventions, but the chips communicate with the CPU at twice the speed of DDR. The slowest DDR2 is therefore capable of 400 million transfers per second, and is designated DDR2-400, or PC2-3200. As you'd expect, DDR2 goes up to DDR2-800, also known as PC2-6400, and above this there's a high-end part, based on 266MHz chips, to give DDR2-1066. Its PC-rating is rounded down to PC2-8500 for convenience - its peak bandwidth is more like 8,533MB/sec.
DDR3 extends this process, running the I/O bus at four times the speeds of DDR - so the basic part can handle 800 million transfers per second, earning the labels DDR3-800 and PC3-6400, with faster chips being named accordingly.
The maximum standard RAM speeds approved by JEDEC - the body behind the three DDR standards - are DDR-400, DDR2-1066 and DDR3-1600. You may also hear of modules with higher speed ratings, such as DDR2-1250 and DDR3-2000, designed to run at overclocked speeds in enthusiast motherboards.
Edited by dc3, 03 January 2011 - 01:45 PM.