The MacBook Air is not very upgradeable. The RAM is soldered on and can't be upgraded after purchase (you need to order it with more RAM from Apple if that's what you want). The SSD is essentially a custom blade with a special connector that isn't sold anywhere else and as of right now, you can't buy a compatible after-market drive to increase your storage space. However OWC/MacSales usually makes compatible drives for the MacBook Air and will probably start selling one for the 2012 MacBook Air soon.
In short, the MacBook Air is not what you want if you can't afford a custom model with the hard drive size and RAM you expect you'll want in the future. The MacBook Pro uses standard hard drive sizes and connectors and its RAM is user upgradeable as well, I believe.
If you get an Air, you will probably store large media files on an external (or network attached) drive. Music and movies tend to take up a lot of space. Apple products are not the kind you buy and then upgrade later, unfortunately.