While it is very unlikely that there will be a need to recover using this backup, not having it in place is very foolish. Should the migration fail or deliver undesirable results in any way, the ability to go back to a known good state is vital. There is nothing worse than discovering months after the installation of the new server that you have lost something and have no way to recover it. This way, you know you have a good copy you can go back too to either reset the clock or recover lost data.
You can either perform a normal backup using the SBS 2003 backup tools or you can manually produce a backup with the NTBackup software provided on SBS 2003 to ensure that you have captured all files. To start NT Backup, go to the All Programs menu on the Start menu, select Accessories, System Tools, and then Backup.
In the backup tool, select all the drives that have data on them, as well as the system state and the Microsoft Information Store for Exchange...