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I want to upgrade my pc's memory and crucial.com tells me that the compatible types are:

Memory Type: DDR2 PC2-8500, DDR2 PC2-6400, DDR2 PC2-5300, DDR2 (non-ECC)
Maximum Memory: 8GB

Does the speed matter? My local shop sells 4G DDR2-800. If I get two of these, will the pc accept them?

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up vote 5 down vote accepted

Of course the speed matters, it determines how fast the memory can operate. You (usually) want to get the fastest speed that you can (limited by the motherboard and price).

Yes, you can put slower RAM in; the system will reduce its RAM speed (not the CPU or other components) to compensate.

If your system can handle up to DDR2 PC2-8500, then it can run the RAM at up to 1066MHz. If you put in DDR2-800 (PC2-6400), then the memory will only run at up to 800MHz. Worse, if you have other RAM modules installed that are faster, then they will all be reduced to the speed of the slowest one.

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thanks. you can see the system specs if you click on the link to the crucial.com site in the question. its a pretty old pc but it only has 2GB of ram at the moment and i need more. –  mulllhausen Sep 23 '13 at 1:22
    
@mulllhausen - Do you have a 64bit OS to take advantage of the greater amounts of memory? If not, you may want to look into that as well, assuming the CPU supports it. Otherwise, you are limited to <4gb ram utilization. –  nerdwaller Sep 23 '13 at 1:26
    
@nerdwaller - yeah i'm running debian 7 amd64 –  mulllhausen Sep 23 '13 at 1:28
    
@nerdwaller, yes, the OptiPlex 745 comes in a variety of configurations, but they are all 64-bit (though they use the archaic label “EM64T”). –  Synetech Sep 23 '13 at 1:33

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