I decided to answer my own question, and I bought an Airport Express today. Talk about one capable device. I have my G4 connected to my AEBS via ethernet and the Express downstairs. The Express connects to the AEBS wirelessly and extends my wireless network. Additionally, I can plug in a computer or hub to the Express and any computer connected gains an IP address from my AEBS without having a direct connection. iTunes in my G4 (no Airport card) plays through the Express just fine. Basically, it does everything I want it to do -- act as a "wireless cable" to downstairs, extend my wireless network, and play music from iTunes computer on my network.

Kudos to Apple, yet again.