Fedora Core internet sharing
The easiest to setup and use is firestarter, since it will use masquerading to allow the computers on the inside network to go onto the Internet without having to use logins and passwords - the PCs are just set up as a "Direct Connection to the Internet". Download and install it, then run "firestarter" to configure it, making sure to check the box that asks if you want to share the internet connection. You may need to manually start it by running "/etc/rc.d/init.d/firestarter start" the first time.
A second option is to turn on Squid in RedHat core. However, this is more complex to setup and only runs a proxy service, which means the users have to have a login and password. If you elect this option, email me and I will help you, since I've done it already, although for home use I prefer firestarter.