the 4th one
actually, there is one more...
http://www.mp3tunes.com/
until apple gets there too
So I think the choices are clear.
You can go with an external hard drive.
You can go with a Server and stream your music (if you have another computer).
You can go with a higher compression too (if you don't notice the difference) but if you do collect a lot, chances are that this still won't be a solution as the Air's current internal hard drives aren't great for bulk... not if you add movies and tv shows to the mix.
I think we've covered all the possible choices to help you. Anyone have a fourth suggestion?
actually, there is one more...
http://www.mp3tunes.com/
until apple gets there too