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I have an Asus X52JC laptop. I have a 2 GB 1066mhz stick, and would like to add another 4gb. If I add 4GB DDR3 1333MHz memory, will the new memory be compatible with the motherboard and with the old 2 GB memory?

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It should be, and the motherboard will slow the faster RAM down to match the speed of the older RAM. As a matter of course, I'd put the 4GB RAM in the first slot so that it's used first, but this isn't much of an issue these days. – user3463 Nov 27 '11 at 23:43
up vote 2 down vote accepted

Yes, it will be. The rated speed of modern RAM is simply the maximum clock speed it can support. It will have no problem supporting lower speeds. The motherboard will select the fastest speed that all sticks support.

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