You may be thinking about old rules about such. Or that old desktop you had that required pairs. Not all machines have those rules.
When I checked with Crucial.com for adding memory to my laptop I got a response that I could add a 512MB to my already 256MB for a total of 768MB. However, I thought you could not add a 512MB to a 256MB already installed as the system may not recognize the added memory. Was this wrong information I was given?

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