SO, are we talking about clustering?
This question is so much interesting to me.
I've think about use a separate VPS for each kind of task (apache, mysql, mail, dns)
It may be done using iptables redirection of the ports, and also rsync.
Doing that you can save all data into a main machine, but use the external copy of your resources.
Anyway, that is not an easy task, and the cluster option is a lot better than simply patch and redirect.