I recently upgraded my ThinkPad to 4GB RAM. However to my surprise I learnt that 32-bit OSes can only access 3GB of the RAM. Sad, isn't it? But, here's how to get happy –

Linux is to the rescue, if you are running a recent variant of an Ubuntu system, just install the PAE version of the kernel, reboot your computer, and you're done

1. First check if your CPU supports PAE, run “cat /proc/cpuinfo | grep pae”, any output is success.
2. Run this command to install the PAE enabled kernel-

sudo apt-get install linux-image-generic-pae linux-generic-pae linux-headers-generic-pae

Enjoy!