Is the router not a wireless one? If not, then you can still share the connection wirelessly, but you will have to leave your PowerMac on 24/7. Of course, this isn't such an issue with OS X :)
If you go into the Sharing section of System Preferences, go to the Internet tab and you can then share the connection from the ethernet port on the PowerMac via the Aiport card.
Should then just be a matter of selecting the PowerMac from the Aiport dropdown.