The best way to determine what you need to back-up is to walk through a scenario where your server's hard drive is no longer booting up.
What data would you require to bring it online as fast as possible?
This is a basic template (tailored to *BSD) that I use:
Files:
/etc/ssh
/var/log
/var/cron
/etc/ipfw.rules (ip tables in your case)
/etc/syslog.conf
/etc/motd
/etc/ssh/banner
/etc/shells
/etc/profile
/etc/login.conf
/etc/group
/etc/hosts
/etc/passwd
/etc/sysctl.conf
/etc/resolv.conf
/etc/rc.conf
/etc/rc.local
/etc/crontab
/etc/master.passw
Dirs:
/var/cron
If you are running other services; you should back-up those configurations as well.