If you edit the files inside
~/.config/autostart/ they are written in this way:
Comment=Sync your files across computers and to the web
Exec=dropbox start -i
The last line is the one that specifies if the app is enabled or not (check it by yourself with the
gnome-session-properties enabling and disabling an app and noticing the change of that option in the file).
Then, you only have to copy all the files inside the folder to make a backup of that configuration.