I'll take a shot at this. I'm new to Macs, but I just got my new iMac G5 to connect to my router (11b ) with a static IP address this way:
Go to System - Preferences - Network.
Select TCP/IP and set the Configure IPv4 to "Manually" then set the following:
IP Adress: [use the static IP address you want]
Subnet Mask: [I'd use the same subnet that the rest of your machines are on]
Router: [Use the local IP address of your router, not the WAN IP that your ISP may give you]
DNS Servers:[You should be able to find these from your router's login page, under something like "Status"]
That should do it. Sorry if things don't quite match your set up, since I've simply copied what works in my limited experience. For some reason, using the "Using DHCP with manual IP address" setting did not work for me, and I had to go whole hog into the "Manually" method.