You can use memory modules of different speeds as long as they are faster than those specified for your computer. It is recommended to use modules of the same speed as those already installed in your system. If you do mix speeds, the modules will all run at the speed of your slowest modules.
For example, for a DDR2 667MHz system you could add 800MHz modules, they will run at the 667 speed.
You cannot mix DDR, DDR2, DDR3 in a system as each technology requires a different type of socket.
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Speeds available from INTEGRAL MEMORY