Memory Clock and Input/Output Clock
Since there must be communication with the other parts of the computer, such as the CPU and the graphics card, RAM must operate at a certain frequency to enable this communication (input/output –I/O- frequency.)
The memory has a main clock, which is run at its own speed (DDR), doubled (DDR2), or quadrupled (DDR3), to get the IO clock.
These main clocks frequencies are 200, 266, 333, 400 or 533 Megahertz
If you do the math, I/O memory for DDR2 RAMs are two times of the memory clocks (400 / 200 = 2) whereas for DDR3 RAMs, they are four times (800 / 200). So, the DDR3 RAMs work twice fast as the DDR2 RAMs.
With the introduction of the Intel’s Bearlake platform (P35 platform) DDR3 RAMs began to receive more attention. With the Core i7 processors, which work only with DDR3 RAMs, DDR2 technology is underway to be history in the coming years. It was thought to be one the way out sooner, but the prices haven't come down enough yet, while DDR2 is now dirt cheap.