I have the airport express. With it I am able to wifi directly from my iMac to the express unit and stream iTunes to my stereo. It works well, but not great. According to the apple helpdesk it wasn't meant to be used this way. They said, the Express is meant to be used with the Extreme or some other hub device. The wifi signal that iMac outputs is not quite strong enough to gap large distances to the Express. I have them both in the same room, so it usually works fine. I have the same Express hooked up to my printer and I wifi to my printer that way via USB.
If you want to have your computer in one room and send your music to another another room, you'll need the Extreme (or some other hub device) to boost the signal to the Express. So, as said by Daddy Elmis above, you cannot stream music via the Extreme "directly". You need the Express, too.