Trouble sharing dialup connection with PCs over ethernet

It all works fine unless I start the firewall on my ibook. How can I configure the firewall to block unsolicited incoming connections without blocking internet access on other machines on my network? I'd rather not allow specific services to pass through in the firewall as that pretty much defeats the purpose of having it turned on.

I'd like the ibook to act like a regular off-the-shelf router but I seem to be missing something. Is the OSX firewall just an all-or-nothing system that applies identical rules to all network interfaces?

The ibook's running 10.2.8, connecting to the internet using the internal modem. The ethernet port's plugged into a small switch along with 3 Windows XP Pro machines. It all works fine as long as the ibook's firewall is turned off but, with the firewall on, the only things that seem to work are DNS querries and pings.

Any suggestions?