DDR2 - Double data rate 2 - A new version of DDR which is twice as fast as original DDR.
DDR2 is the latest in high speed memory. These speeds are available: PC2-3200 (DDR2-400), PC2-4200 (DDR2-533), PC2-5300 (DDR2-667) Memory. Note: DDR2-xxx denotes data transfer rate, and describes raw DDR chips, whereas PC2-xxxx denotes theoretical bandwidth (though it is often rounded up or down), and is used to describe assembled DIMMs. Bandwidth is calculated by taking transfers per second and multiplying by eight. This is because DDR2 memory modules transfer data on a bus that is 64 data bits wide, and since a byte comprises 8 bits, this equates to 8 bytes of data per transfer. Some manufacturers label their DDR2 sticks as PC2-4300 instead of PC2-4200, and PC2-5400 instead of PC2-5300. At least one manufacturer has reported this reflects successful testing at a faster-than standard speed, whilst others simply use the alternate rounding as the name, as described above.