If you want to keep an Air for more than a year, and keep its resale value, 4GB is a must. 2GB is the bare minimum for Lion now. If you don't get 4GB you'll regret it.
I've never resold anything (I just give stuff away). What I'm wondering about is whether I'll be tearing my hair out when on Skype or YouTube, though even my original white eee netbook could handle both on a quarter of the ram. I miss that little netbook, come to think of it!
Right now Lion will run on 2GB, and will do for browsing, iTunes etc. But you can't upgrade the RAM later, just think in two years 10.8 may be out and 2GB will be low. I'd seriously consider 4GB.