I'm going to assume you haven't purchased the MBA, and I was going to suggest upgrade the RAM yourself, but I just read that it is not possible with the air. I suppose cause it's so thin, but that does suck since it is always cheaper to do the upgrades yourself.
RAM is one of the biggest reasons why I haven't gotten an Air yet and am hoping that Apple boosts for the upcoming revision. Count me in as one of the many hoping for an 8GB option when the new revision comes out...whenever it may be. Granted, my tasks aren't as taxing as some of those mentioned here, but I just want to be able to use a piece of gear for as long as feasibly possible.
YES, RAM is much more important than many things people care about when buying computers. Hell, 2 GB RAM with an i7 would be a much worse experience than 4 GB RAM with an i5. I'll try to explain why in a few, easy to understand sentences:
Every time you open a program, look at pictures, listen to music, watch movies, etc. those things need to be stored somewhere on the computer where they can be easily accessed quickly. See, opening things from the hard drive is a slow process (made a lot better by the SSD the MBA has, but still not exceptional), so RAM is used to store temporary things that you're currently using.
So, if you were watching a movie, had 8 applications open, and were browsing a resource-heavy website like tumblr (lots of images/audio/video), you would need to store all of those applications, media, etc. in a place this big:
It works, but it frequently gets filled up which means the computer has to do extra work clearing it often, moving its contents around, and using the SSD instead, which is slower. Now, if you had twice as much room:
You could use many, many more thing without even once feeling a slow down! This also means your computer can boot faster! Though, I want to point out that 4 GB is pretty low right now and 2 GB is bordering on 2007 territory. You should wait (if purchasing now) for the new MBA's coming out that would hopefully support 8 GB RAM. 8 GB RAM allows me to run Windows, Mac OS X, and Linux at the same time (virtual machines) without any of them slowing down! Seriously, 2 GB is really low today; don't go for it.
Just to give you an idea of how big 8 GB is compared to 2 GB: