I would go with ipcop. Smoothwall is risky. They have a proprietary version and a GPL'd version. They tend to focus much more on the proprietary version at the expense of the GPL'd version. IPCOP is totally GPL'd and so the focus is on that product. I don't think any of the other products are decent competitors.
As far as speed, you need a switch for one thing. Most things that today say they are hubs are in fact switches. About the only use for a hub is if you are running a packet sniffer of some sort (something where computers read the packets sent to other computers on the net). A hub works by getting a packet from one computer, and echoing it to every other computer on the hub, so every computer receives every packet, and it is left to the nic's to filter out the traffic. A switch routes the packet only to the target computer, cutting down on excess traffic and increasing speed. Much superior for most users.
If you go with smoothwall, ipcop, or something similar, you will need to attach a switch or a hub to one of the nic's (the blue nic) to connect all your pc's to it, so it doesn't really save you much money. Another thing to consider is that ipcop or smoothwall cost more to run in a year than the cost of a cheap router (computers require a lot more power than dedicated router units), so I wouldn't use ipcop to save money.
Finally, for security purposes, you need to have a router of some sort. IPCOP is a router (as is smoothwall), but you can also use a dedicated rouer like a linksys/netgear/dlink/etc. This creates a hrardware firewall between you and the outside world. DO NOT run anything else on the ipcop box. It's just ipcop, it just runs as a firewall/router. If you try to run other things on that box, you now have a very insecure network setup. For reasons of security, it is critical to have some kind of HARDWARE firewall between your pc's and the outside world... something that does NAT and blocks all the ports etc.
So if you need the old junker to do something else, like word processing, web surfing, etc... i.e. function as a general computer, you cannot use it to run ipcop or smoothwall or any similar program. It is highly insecure and asking to get malware or hacked.
People use ipcop over a regular router because it offers a lot of features the router does not. It allows for traffic shaping, the use of a caching proxy, network analytics, a more secure system especially if you have green/blue/orange/red zones, http filtering for viruses and/or ads, removal of ads automatically from all websites, etc. There are nice features associated with ipcop, but a regular router is ok for security too. Some kind of router is absolutely necessary though.
