Ubuntu Security Basics
Short of creating my own do-it-yourself guide to Ubuntu security, Ubuntu forum Master Roaster, bodhi.zazen, wrote one already. The Ubuntu Security forum entry includes information about password strengths, anti-virus, firewall, adware, root-kits and IDS, among others. I highly recommend that you check it out and see if you did not miss anything when you hardened your server.
[image from Ubuntu Forums entry]