You should use the exact same memory, especially speed and latency to run well. In fact if you can get the same brand and model series and the two memory cards you run are close enough matched your system may run in dual channel mode which should further improve performance. If the price of a matched set is not too high consider getting that instead since it should definitely run dual channel and you probably then have the option for faster memory than what was supplied with your system. You can use Crucial.com to scan your machine and find out what your best options are for memory (even if you purchase elsewhere).
You have to be a little bit crazy to keep you from going insane.
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