I have a Toshiba A100, which I upgraded to 4G of RAM. The hardware startup indeed shows 4G of RAM, and I recently installed Windows 7, just to see how it behaves on it. So far so good, it displays 4G of RAM. Not that I tried to use it all, but it displays it. Previously under XP, I also would see 4G of RAM.
But under Ubuntu 9.10 (32 or 64 bits), it only displays 2.9 G of RAM. And my kernel is the "pae" compiled one, which is supposed to do the trick to work around the 32-bit CPU limitation.
How can I get Ubuntu to fully use my 4G of RAM ?