From Kingston website:
DDR2 memory was developed by JEDEC, the memory industry's standards organization of which Kingston is a member company. DDR2 memory chips and modules use a naming convention similar to that used for the current DDR memory modules, but are not backward compatible due to incompatible module connections (number of pins), voltage and DRAM technology. DDR2 memory modules have a different key or notch than the same-sized DDR modules to prevent their insertion into an incompatible memory socket.
DDR2 memory chips will be available in Fine-pitch BGA (FBGA) chip packages for improved electrical and thermal characteristics. In addition, DDR2 memory chips will incorporate On-Die Termination (ODT) to minimize memory signal reflections at high speeds, thereby improving timing margins. DDR2 memory chips will come in capacities up to 4 Gigabits, allowing for higher-capacity modules.
Key DDR2 features:
* 1.8 Volts operation, reducing power consumption by about 50 percent
* Memory signal termination inside the memory chip ("On-Die Termination") to prevent reflected signal transmission errors.
* Operational enhancements to increase memory performance, efficiency and timing margins.
* CAS Latencies: 3, 4 and 5
* 400MHZ and 533MHz and capacities up to 1GB currently.
http://www.valueram.com/ddr2/default.asp