A development virtual machine can be really handy. It gives you a sandbox of sorts where you can feel free to test and experiment knowing that in a worst-case scenario you can just delete the VM and start over. It can also be a great way to practice server configurations and sketch out "real-world" server setups. Here's the process I follow to setup my Ubuntu 10.04 LTS Drupal development VM in VirtualBox. Aside from the VM-specific steps, these instructions should work for a regular Ubuntu server (VM or not).