I'm not exactly sure how you can have the Linux box do the firewall for your whole network without using it as the router. You could use it as an IDS box and then at least you would know and there are some controls. For that I would reference http://www.internetsecurityguru.com/.
It is specific to FC2 but you would get the idea. If you want some documentation on iptables, try http://www.netfilter.org.
That should give you some places to start. If you want to take a leap of faith, you might try replacing the Linksys with a Linux box, using it as a router and running IDS. Then you would be able to play with the firewall, get alerts when someone is trying to get in and learn a whole bunch in the process. You would not need a big fancy box to do this with . Even an old 486 or equal would work. If you are using the Linksys to do PPPoe to connect to your ISP the nix box would do that just fine. Good luck and repost when you get a chance.