what is difference between DDR and DDRII RAM?

Answer 1

In the simplest yet most accurate terms :

DDR2 uses 2 channels to transfer data instead of 1 channel like regular DDR.

This means that although transferring at the same speed it moves twice the amount of data at the same time.

Speed = Width (1 or 2 channels) x Frequency (400Mhz, 533Mhz, etc)
...therefore DDR2 ram running at 533Mhz is in effect like having a regular DDR module running at 1066Mhz.

regards,
Philip T

Answer 2

DDR SDRAM is a straightforward evolution from SDR SDRAM. The big difference between DDR SDRAM and SDR SDRAM is that DDR reads data on both the rising and falling edges of the clock signal, so the DDR module can transfer data twice as fast as SDR SDRAM.
While DDR has a limited clock rate, the evolutionary changes to DDR architecture enable DDR2 to achieve speeds beyond of DDR, delivering bandwidth of 5.3 GB per second and beyond! Because DDR2 is able to operate with faster bus speeds, your memory doesn't hold back the performance of your processor.

motherboards are built to support only one type of memory. You cannot mix and match SDRAM, DDR, or DDR2 memory on the same motherboard in any system. They will not function and will not even fit in the same sockets.