Am running Xubuntu 12.04 on an old Compaq with 512mb memory which ran fine, I did however add a line to the end of the file /etc/sysctl.conf

vm.swappiness = 10

Also by adding noatime flag in /etc/fstab disk access was faster also...

Here is an example of my fstab file:

UUID=1d508e27-4068-45f9-88d4-f82928837892 / ext4 errors=remount-ro,noatime 0 1
# /home was on /dev/sda5 during installation
UUID=01d2649b-89b9-4cc7-8886-177d25a7868f /home ext4 defaults,noatime 0 0