[ubuntu] Some RAM, but not all, is missing

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http://ubuntuforums.org – I'm working on my girlfriend's laptop. She's got an Acer Aspire 5100, which claims it comes with 1GB of RAM in 2X512 configuration, but only 768MB shows up in Ubuntu. I just opened the back and there are indeed a pair of 512's in there. I'm running MemTest right now, but it too only shows up as 768MB. I'm guessing one of the two sticks of RAM is somewhat faulty. I'm thinking of buying a replacement stick, swapping out one of the sticks and seeing how much RAM shows up, and if it looks like I pulled the non-faulty one, going back in and swapping it for the flawed one. Any better ideas? I've never had RAM do this before. (Hardware)