UNIX for Advanced & Expert Users

We've got a FTP server that's open to the public network and its running on Suse SUSE Linux Enterprise Server 11 (x86_64) SP2

Now, since it's an FTP server I can't disable that service, but how else do I harden this server from attacks from outside?

I am thinking of disabling the firewall and only allow communications with FTP ports, and also changing the Selinux to enforcing mode.

Anything else anyone can think of?

maybe you can replace ftp by sftp ?
disable firewall ? the real problem with FTP is that service sends all data in clear text. If you allow only FTP service ...imho it is a poor protection. SeLinux is a good start. What about chroot all ftp users ?

On SUSE I would use a good ftpd, try AppArmor, leave that firewall on and only allow the required ports and maybe also use TCP_wrappers (it can't hurt).

Have a look at the OS-hardening tool Bastille.