Scaling out your Drupal and Database Infrastructure, Affordably - Arjen Lentz
It's common for 'simple' Drupal systems to have a combined server for both the web server and the database. That's an easy way to start, but it can become nasty to maintain.
Obviously, any maintenance or upgrade work (OS security updates, for instance) requires scheduled outages. While for some sites that's ok, for many it's not. And what about backups, and the possibility of (parts of) the system failing?
You want to keep things simple, as low budget as possible, and preferably forget about it for most of the time - without it coming back and biting you later.