Does anyone know a chart that shows compatibilty between core modules and dotnetnuke versions?
The IFRAME 3.1.0 came with the dotnetnuke 4.0.3 Install, so thats a safe bet. (of course the guru gave you a one line fix for the current module)
As far as backing up thats a much larger topic. But the key parts are for a basic installation would be:
Backing up the file system and structure.
Backing up the SQL database.
Being able to RESTORE from those backups.
If you're in a shared hosting environment, those backups would include the other users/portals sensitive data. In your case ask your host if options are available.