Making Debian Linux Recognize More Than 4 GB of RAM – (BIG MEMORY)
If you have a server with 8GB of RAM and Debian could only detect 4GB by default.
Then you will need to install this package called “linux-image-2.6.18-5-686-bigmem”

This package provides the binary image and pre-built loadable modules for Linux kernel 2.6.18 on Pentium Pro/Celeron/Pentium II/Pentium III/Pentium 4 with 4-64G RAM machines.

How to Install

apt-get install linux-image-2.6.18-5-686-bigmem
reboot

For DEBIAN Lenny
It uses package: linux-image-2.6-686-bigmem
This package depends on the latest binary image for Linux kernel 2.6 on Pentium Pro/Celeron/Pentium II/Pentium III/Pentium 4 with 4-64G RAM machines

How to Install

apt-get install linux-image-2.6-686-bigmem
reboot