NAT32 is also an option.
Win98SE/ME's Internet Connection Sharing would be the worst to choose. I've yet to see someone say that they're really happy with it. More often it's reported that it's buggy, slows down the throughput, etc.
All software options will require that one of the machines have a second network card installed, and will slow that system down somewhat. Everyone reports varying degrees of performance loss on the router/firewall computer though, so I couldn't say for sure what you would see.
Best option for ease of use and performance once you get cable is a Cable/DSL router from Linksys, Hawking, Netgear, Asante, SMC, etc. They're less than 150 dollars, some near 100. The Asante FR3004LC comes with a serial port for attaching an external modem (meant to be a backup to the main service), so you could get it now, use it for the 56k connection, then plug the cable in when you get it, no changes needed for the rest of your network.