Question about Ubuntu server though -
I’ve got it up and running the file share ok at this time. I’m curious though as to what it would take to have this machine acting as a gateway between the internet and the home workstations.
Currently, the network is set up as follows - cable modem to router, router feeds the three workstations and the Ubuntu file share.
Can I have it going cable modem to Ubuntu, Ubuntu to the router - router to the workstations? I have two NICS in the Ubuntu box, so one could act as the incoming from the cable modem and the other outgoing to the router.
You’ve already got a router. It has a firewall in it (probably). Why add another gateway?
It's possible, and many people do it. However, since you'd need to configure your own firewall rules, this requires a good understanding of TCP/IP routing and of the iptables command. There are some packages acting as a GUI on top of iptables, but a good understanding of this command is critical to the operation of a gateway. So read up the tutorials on it and experiment, since you have two NICs.
Also, there is one good reason for which you might NOT want to use a PC as a gateway: reliability. A PC has rotating fans and hard drives that make its reliability lower than that of a full solid-state gateway. And when you gateway breaks down, you can't even look online for help!
Me, I use a Linksys WRT54GL router (designed to be easy to hack) and I flashed it with the free DD-WRT linux-based home gateway software, which has an excellent web interface AND a command line. The router has no moving part and can be bought new for less than $50.
If you want to experiment with Linux as a gateway, that's the solution I recommend.