The other solution is to connect a wireless router to your computer downstairs, and to connect a wireless ethernet bridge to your Gamecube. Both of these will run you around $200 (together).
Our cable line is downstairs in our house as well. i haven't gotten the wireless ethernet bridge yet, but my mom has her iBook connected wirelessly and has access throughout the house (and through the backyard).
A possibly cheaper solution would be to get the modem adapter, and PSO through dialup. i played a few games through dialup and there's no major difference, though i'd recommend going wireless if at all possible.