I'm assuming you know that you really should get either a Macbook Pro, iMac, or Mac Pro for After Effects and you're only asking incase you need to use it on a pinch on the air....but in case you don't:
You should really get a machine with at least 8 - 12 gigs of RAM for After Effects. It snarfs up 22 gigs of RAM on the desktop in my sig (that being said I don't use it often and haven't made use of proxies for HD video).
That being said it is run-able on the air in a pinch, but I wouldn't recommend you do a lot of rendering with it.