RAM chip
Despite sharing the same basic size and dimensions, DDR and DDR2 SDRAM modules are not interchangeable. This is due to physical differences that manufacturers build into the motherboard’s RAM slots.
Problems
DDR2 chips use a 240-pin interface to connect with the motherboard, while DDR uses a 184-pin interface. This makes it physically impossible for them to occupy the same slot.
Misconceptions
Part of the misconception that different types of DDR are compatible comes from the fact that different DDR speeds are often backwards-compatible; that is, a faster version of DDR will usually work in a motherboard designed for slower DDR.
Types
DDR speeds include 200, 266, 333, 400 and 466 MT/s (Mega Transfers/second). While DDR2's design makes it incompatible with DDR, it also allows it to retrieve more data than DDR, reaching speeds of up to 1066 MT/s.
Considerations
If you’re using DDR and intend to upgrade your system, adding faster DDR will not result in a performance gain, as the faster memory will run at the same speed as the memory for which the system was designed.
Solutions
The only way to move from a DDR-compatible system to a DDR2-compatible system is to upgrade your motherboard to one that supports DDR2, while making sure that your processor and other components are compatible with the new board.
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