What is the difference between PC2100 (DDR266), PC2700 (DDR333), and PC3200 (DDR400)?

Although these are all types of double data rate (DDR) memory, the varying numbers refer to the different speeds of memory your computer was designed for. Crucial no longer carries PC2100 (DDR266) memory, but PC2700 (DDR333) memory and PC3200 (DDR400) are backward-compatible with PC2100.

Memory is designed to be backward-compatible, so generally speaking, you can safely add faster memory to a computer that was designed to run slower memory. However, your system will operate at the speed of the slowest module or system component (the "lowest common denominator" effect).

One thing to keep in mind is that the memory does need to be the same type-for example, SDRAM cannot be mixed with DDR, and DDR cannot be mixed with DDR2. We recommend that you use the Crucial Memory Advisor™ or System Scanner tools to find the right memory for your computer.

For more details about memory speeds, see Memory Speeds and Compatibility.

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