How to provide fail-over capability for servers?

[Stephen Allen]

I'm completely new to this so some of my ideas may sound ludicrous... 
please put me right where necessary!

I will have 3 net-facing servers which must be available 24/7.  I had 
planned to have an additional box located in a different building on the 
same site (in case of part power-outage) that will run VMware server and 
host all the fail-over servers.

What options are there for providing automatic fail-over capability?  I 
had thought of running a cron job every minute that would test for the 
existence of the other server and assume its identity if it couldn't get 
a reply, but it feels like a 'sub-optimal' way of doing it.

Can anyone give me some ideas I should be looking at?

Many thanks,
Steve :)