DDRx Memory: Memory Clock vs I/O Bus Clock?
DDR,DDR2 and DDR3 memories follow the DDRxxx/PCyyyy classification.

The real clock of DDR, DDR2, and DDR3 memories is half of the labelled clock speed. Therefore DDR400 memories work at 200 MHz, DDR2-800 memories at 400 MHZ.

The bus clock rate is the actual speed of ur FSB(The FSB connects the processor (CPU) in your computer to the system memory).