First you need to be able to login to your N800 as root.

just install the openssh package and it will ask for new root pass
you have to ssh root@localhost to get root on the N800

Now, from your linux box, lets say you want to backup your n800 to /backups/n800/
from a terminal type

mkdir backups/n800

now login to your n800
Make sure you are not connected to the n800 via USB

ssh root@n800ip

where n800ip is the ip address of your n800

I didn't have any luck with rsynch or scp because of looped symlinks

so use the tar across network method

tar -cf - / | ssh regx@linuxboxip tar -xf - -C /backups/n800

If you get errors you might have to backup the directories one at a time

tar -cf - /usr | ssh regx@linuxboxip tar -xf - -C /backups/n800

to backup the user dir

tar -cf - /home | ssh regx@linuxboxip tar -xf - -C /backups/n800

for the home dir ...etc