This link may help.
http://support.apple.com/kb/HT4433#3
MacPro, believed to be 2008 model, Quad Xeon, 10.6.4
Shipped, used, to buyer with one memory module. Buyer had already purchased additional RAM modules which were installed prior to the machine ever being booted.
System Profiler recognizes the two new modules and ignores the original 1.

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