I have my PC networked with my G5 no problem via firewire, internet connection sharing even works over it.
All you have to do is add the FireWire port to your network settings, do this in System Pref. Click on the network button and in the show combo box select Network Port Configurations. Click the new button, call it whatever and select Built-in FireWire.
Then all you have to do is connect the two machines, give em both IP addresses in the same subnet e.g. Mac 192.168.2.2 subnetmask 255.255.255.0, PC 192.168.2.3 subnet 255.255.255.0 dont worry about dns or anything. Turn on file sharing on both machines and you are golden.
Let me know if you need any more specific details. When I plug the mac into the PC XP wants to add a new device, I just cancel, it still finds the firewire network connection.