Looking at the board specs you have onboard video unless you installed a APG graphics card. The onboard video could be set to 128MB of shared memory and this will make up the difference between 384 and 512MBs. I bet if you went into your BIOS and changed the amount of video memory to 64MBs it would also change the total amount of memory seen by your computer. Also double check to see if the BIOS is reporting all 512MBs. The memory is probably working fine, the shared video memory is probably set too high.
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