Well you gotta have a backup strategy. I'm talking about the primary machine here, I assumed that would be obvious but maybe not. You build your failover strategy into your architecture - there's lots of ways to do it - I use Postgres so I would favor something based around log shipping.
And uptime is important, so you want to have that secondary running and ready, with a proxy in front of everything so you can switch as soon as you detect a failure. That's three hosts, plus your alerting has to be separate too, so that's four. Now, to orchestrate all this, we'll first get out Puppet...
If you're going from one machine to two, and you add an automatically failover mechansim, chances are your load switching mechanism is going to cause more downtime than just running from your single machine, and manually switching on failure (after being paged).
