Start with documenting everything. Get a good high level overview and then dig into all the details.
Now deceide how quickly you need to have things back up and running. Do you need a hot site? Cold site? Do you just need the data and config or do you need full hardware redundency?
For most of my sql db's there are server images, registry backups and regular sql backups, the sql .bak file is then distributed to an off site server and then backed up to tape overnight.
The details of exactly what you do will depend on what kind of security you require and how quickly you need to restore the systems.
In any case, PLAN TO TEST YOUR BACKUP PLAN. A disaster is a really bad time to find out it didn't work.