For example, your machine has PC2-4200 (533 MHz) DDR2 memory in it, and you have another module of PC2-5300 (667 MHz). Can you use it?
Your motherboard’s memory speed will always be set to the speed of the slowest module you have installed. As long as it is the appropriate type of module for the machine, the PC2-5300 module *should* clock down to 533 MHz OK, but this relies on the SPD (Serial Presence Detect) programming being done properly on the module by the memory manufacturer. This data tells the motherboard the identity of the memory module, and the range of speed, voltage and latency settings that can it be used with. Some cheaper memory doesn’t have complete SPD programming and these will cause you problems.
This is machine dependent, some chipsets restrict the amount of faster memory that can be used and the number of Ranks of memory that can be used. You can use the Kingston memory configurator to check the specs of your machine by model number. Also remember that memory is not backwards compatible across types – you cannot use DDR3 memory in a machine that requires DDR2.