In Debian, use the apt-cache search command to locate a recent linux-image package for a kernel for a 686 processor if you want support for up to 4 gigabytes. If you want support for up to 64 gigabytes, look for a kernel that ends with 686-bigmem. These kernels will enable support for 64GB. Unfortunately, a generic kernel for only 4GB does not exist.
aptitude install linux-image-2.6-686-bigmem
That’s it now you need to reboot your system and you will be able to install up to 64GB of RAM to your system.