Do you need to backup the whole registry or jsut the keys your editing? It doesn't sound like you need to do the whole registry file just the keys your going to edit instead. If so then just export them to a reg file from inside regedit. If you mess up just double click this registry file and it will revert the keys back to what they were before.
If you want to backup the entire registry I recommend doing a System State backup with NTBackup. Ghost can only image the whole drive or partition, not just the registry.