What do i need to backup in AD?

just finnished a NT4 -> AD 2k3 upgrade.
I have my DC running on a bran spanking new Dell PE 2800 and I installed Exchange on my old DC/File Server (its a dualie P3 700 w/ 2GB of Ram)

My question is, what should I be absolutly sure I back up incase of failure to keep AD and Exchange users and settings, I installed Exchange on the M: drive of my mail server,and i back that up, so i’m definatly backing up the exchange edb’s and log files

but i’m worried about settings, Schema, etc. etc.

any input would be great, thank you!