Backward Compatibility
DDR2 DIMMs are not designed to be backward compatible with DDR DIMMs. The notch on DDR2 DIMMs is in a different position from DDR DIMMs, and the pin density is higher than DDR DIMMs in desktops. DDR2 is a 240-pin module, DDR is a 184-pin module. Notebooks have 200-pin modules for DDR and DDR2, however the notch on DDR modules is in a slightly different position than that on DDR2 modules.
Higher-speed DDR2 DIMMs are compatible with lower-speed DDR2 DIMMs although the motherboard or CPU memory controller will be bound to the limits of the lower-performance modules.
Read more about this topic: DDR2 SDRAM