PCStats has done an excellent job of explaining the upgrade from DDR to DDR2.
What is DDR2? Well, it’s a new memory standard, as defined by JEDEC (Joint Electronic Device Engineering Council), whose members include many of the major computer memory and chipset manufacturers. JESD 79-2A to be exact. DDR2 picks up where DDR memory currently stops, at 400MHz.

Addtional Memory Links from around the Web:

Click Here to go to Memory  MEMORY