Jaguar will do exactly what you want. In the Network preference pane, I think you need to make sure that Airport is above Built-in Ethernet in the list of port configurations. Then just go to the Sharing pane and turn on internet sharing. The text in the sharing pane says "You are connected to the Internet over AirPort. This connection will be shared with computers connected to Built-in Ethernet."

The only thing about which I am not certain is whether you need to manually assign IP addresses on the wired portion of the network, or whether turning sharing on makes the router Mac a DHCP server. Someone else might know.

Overall, though, setting up what you want is supposed to be simple.

Chris