Updates: All the work you have just done is not worth the effort if you do not keep your programs up to date. New holes are found every week and the crackers stay very much up to date. Visit your distribution's home page often and check what updates are available. Install them immediately! The Mandrake distribution has one of the best solutions I have seen with their MandrakeUpdate program.

Turn off what you do not use! It might be cool to have all sorts of services running on your machine (like http, ftp, finger, etc) but unless you need them, all they do is leave a door open to who ever wants to come in. Edit your /etc/inetd.conf file and comment out (put a # at the start of the line) everything you do not need. Since staying up to date is hard enough, the fewer programs you have to worry about the better your security will be.

How to add a machine into your network later:  It is likely that you'll add systems to your network after you've completed this setup at some point in the future.  Here are the key files that must be updated with your new system and it's IP address information:

    hosts.allow
 

Additional resources: There are plenty of resources available on the net. Most are for the professionals, but they have some relevance to us at home as well.  Below are some pages we feel are critical to home security:

The Linux Administrator's Security Guide The best source of security related info available! Get it, print it out, (178 pages as of today) and read it! Note: it is in pdf format.
Linux Basic Security Nice overall look at security
Linux Firewall and Security Site The most comprehensive list of security sites on the net.
LinuxPowerd.com Another very comprehensive list of security sites, including pages for updates of the more common distributions.