the pc will be the hub/base, connect the connection from the adsl modem to the router, then a cat5 from the router to the pc's ethernet port, the imac g5 should have an airport card if its gonna be wireless, on the pc, go into your favorite browser, and put in the ip address that is in the manual that came with your router, setup preferences, wep, etc there, on the imac, turn on the airport card and it should automatically pick up the signal.
keep us posted.