Does the memory show up at its full capacity in your BIOS/startup post screen? If not then you may need to flash the BIOS to a newer version you might also want to check to see if your motherboard (what make/model is it anyways?) supports 521mb sticks. One thing you can try is when you get to the grub boot screen hit the c
key then boot use the grub command prompt and add the mem=512M to the command when booting the machine. To do this look at the first entry/the one that boot Debian if not the first in your /boot/grub/menu.lst file like this below.
title Debian GNU/Linux, kernel 188.8.131.52-cfs-v19 Previous
kernel /boot/vmlinuz root=/dev/sda2 ro
Now once you have used the c
key to get to the command prompt you would need these commands to boot your system using my entry above as a template changing to yours when you do it.
kernel /boot/vmlinuz root=/dev/sda2 mem=512M ro
Now hopefully it will see and use all your memory if this works then edit the line in the menu.lst and add the mem=512M to it to have it used on every boot.