[ubuntu] Incorrect amount of memory after upgrade

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http://ubuntuforums.org – I'm running Ubuntu 12.04 on a HP Compaq nc8430. I've just increased the memory from 2Gb to the max of 4Gb. In the BIOS the amount of memory showing is 4Gb and memory test passed OK. However, when I check how much memory is installed in the System Details, it is only showing 3.3Gb. Anyone know why this is? Thanks. (Hardware)