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I have a 4096 MB and a 1024 MB DDR3 memory stick. They both have the same clock. I know they won't work in dual channel speeds because they have different sizes. But will they work better alone than together?

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up vote 4 down vote accepted

There is a performance hit, but in most real-world scenarios it should be negligible - you'd probably have to run RAM benchmarks to see the difference.

Also note that all recent Intel-based systems1 support asynchronous dual-channel, which in your case would mean that 1+1 GB would work in dual-channel mode and only the remaining 3 GB (of the 4 GB stick) would be in single-channel mode. This makes the performance difference even less noticeable.
Note: this may not apply to you if you have an AMD-based system, but I thought it was worth mentioning regardless.

Provided you actually need 5 GB of RAM, I'd say go for it.

1 Most systems from 2004 or later, and definitely all that support DDR3 RAM.

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Could you give a clearer definition of "most modern systems"? For example, which chipsets/CPUs started supporting it in home computing? – Bob May 17 '12 at 23:28
@Bob Good question. Intel introduced the Flex Memory technology back in 2004, with the 910 and 915 chipsets. Not sure if AMD has something equivalent, though. Perhaps I should amend my answer to say "all recent Intel-based systems". – Indrek May 17 '12 at 23:36

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