[Hampshire] 4GB on AMD64

Hi all,

Does anyone have any ideas what needs to be done to get Linux to
recognise 4GB of RAM? I've got a box running 64bit Gentoo Linux (AMD),
which I've just upgraded to 4GB, however Linux is only showing 3GB
available (according to top).

The machine's BIOS is displaying 4GB installed, it's just Linux that
only seems to see 3GB.

Thanks,
Sam.