Just a quick post of how to update a low memory system by ssh.
I couldn't upgrade using the panel as I kept running out of memory and receiving some obscure message. I used the following method to update using command line. Basically I killed all the services and ran the update on my 512mb system.
/etc/init.d/psa stopall /etc/init.d/mysqld start /usr/local/psa/admin/sbin/autoinstaller