How to use 4 GB RAM on a 32-bit Ubuntu?

I increased my 32-bit Ubuntu box from 2 GB to 4 GB RAM and found out that the box only recognized 3.1 to 3.2 GB RAM. I then install the server kernel to make it to read 4 GB of RAM and reboot afterwards.
sudo apt-get install linux-server linux-headers-server
However, my nVidia GeForce 8500GT display [...]