I got a new laptop last week and it came with an extra RAM module. I thought it would be fun to have more RAM in laptop than I have in my desktop machine, so I put it in and to my surprise Ubuntu was reporting only 3GB of RAM, even though the machine had 2x2GB modules. I checked BIOS and it correctly said the machine has 4GB of RAM. It turns out that on a 32-bit machine you can address only 3GB using the standard addressing method. There is an extension to work it around, called PAE, but the default Linux kernel in Ubuntu has it disabled. I was afraid I’d have to compile my own kernel, but fortunately there is a package with PAE enabled, so I only had to do:
sudo apt-get install linux-generic-pae
Reboot and woohoo, /proc/meminfo now shows the full 4 gigabytes.