A greater speed DDR module will not work faster, it will be limited to your system's bus speed.

There is no speed gain in putting a faster DDR DIMM in your computer. By doing so you run the risk of incompatibility.

Always match same speed memory in your computer for 100% compatibility.

DDR (double data rate) memory is derived from SDRAM technology. It performs two data operations in one clock cycle - providing twice the throughput of SDRAM. Therefore, a PC266 DDR memory module can transfer data twice as fast as a PC133 SDRAM module. Developed by JEDEC, it is a competing technology with Rambus memory.

For stability and compatibility: Use the correct speed DDR in your motherboard.
Front Side Bus 266Mhz uses PC2100
Front Side Bus 333Mhz uses PC2700
Front Side Bus 400Mhz uses PC3200

There are too many variables to simply assume a faster speed is backward compatible to all lower speeds.

There is no speed gain in putting a faster DDR DIMM in your computer. By doing so you run the risk of incompatibility.