While I too believe that it is a BIOS setting that is giving you troubles, as another suggestion and data point, whenever I install new memory, I run memtest (via the install CD boot: prompt) and let memory tests run overnight [if it finds all the memory installed on the box.]. If it doesn't find all the memory, you want to start looking at the memory sticks, are they seated correctly, etc. Finally, I check timing.
Then, if Linux doesn't see the memory, it is usually the BIOS that is the problem (at least for me where my memory is less than 8 GB). Also, I go over the first couple of pages of dmesg (at boot-up) with a fine tooth comb and understand all the messages, their implications, etc.
If all else fails, I use the* Linux*-ready *Firmware* Developer Kit at:
http://www.linuxfirmwarekit.org/
While it doesn't do a lot with memory, it might find some "issues" with your BIOS .....
Hope this helps ...
Rich