mghz whats the difference

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i know the difference between ddr a sdram, and the old 386 memory cards, but what is the difference in the mghz and how will it effect the performance of my computer. i just installed a 256 card ddr ram 300mghz into my computer to go along with a 512 at 266mghz and everything seems to be ok for now. it seems a little faster. is this a close enough match or should i look for something in the line of exact match. i can get a card of 512 ddr ram with a different mghz rating but am unsure as to compatability and how it will effect the performance