Maybe I should post my questions here.....

I want to setup a redundant server, and need some pointers please. I got two
machines, setup exactly the same way, and I want them to be able to back each
other up. i.e, if the main server fails, the secondary should take over,
without disrupting any network services.

The purpose of the server(s) will be a mail server, SQL server for a medical
practice, thus downtime should be of absolute minimum.

I had a look at the Cluster-HOWTO, and this to an extend could do that I need,
but I'm not sure this is really what I need.

I have got DNS setup on both machines, one being a master and one a slave. I'm
also going to setup, and I also have exim installed on both machines (though I'm
not sure howto make the one take over)

My questions are:
How do I go about setting both servers up to back each other up? Does the one
backup server need to change it's IP address once the main server "fails"?
If email gets send & received via the backup server, and the mainserver comes
back online, how do I get the mail from the backup server to the mainserver?
The main server also acts as the internet proxy / router. How would the backup
server be able to take over these services?