Setting up a hot backup machine

[nabcoengineer]

Hi all. I hope this question has not been asked before.
I have a WEB server running on Ubuntu 8.1 server with ISPConfig, Mysql, PHP5 and I want to set up a separate machine as a hot backup so I can just unplug the main machine, change the IP on the backup machine and get the web sites back up and running in case of a catastrophic failure of the main system. I do have a mirror and a nightly backup of databases and web sites but this does not help if the machine itself huffs.
My question is this, what files and directories do I have to rsync to the backup to make everything work. I figure I need all of the /var/www stuff and all the mysql databases and the /root/ispconfig stuff but what else is there. I have tried so much now that I have to rebuild the backup machine from scratch.
Or... Is there an automated program that will handle this. I can connect the two machines together with a crossover ethernet cable cable to speed things up and not load down the network so it does not matter how much I have to copy.
Thanks in advance.