"So, you're saying get a $129 device (which requires a network drop) versus a $100 antenna. Might work. I don't know.
Or, wait, it doesn't require a network, because it would be hopping network to the Extreme Base Station, is what you're saying? And, does AirTunes work in this configuration too? Hmm, sounds like you have to strategize 1) Stereo placement, 2) lossy network areas...?
Airport express acts as an access point. so you can plug it in at the other end of your house and it will send the signals back to the base station. This is a much better idea then getting another antenna.