I'm looking At buying a MacBook air - most likely the 13" model and unfortunately without spending a lot more money I cant get 4GB of RAM in store. Obviously I can 'build' one on the apple store but I'm not sure about delivery times so is a bit risky. In terms of of a MBA being used for work - web browsing - film watching - FaceTime - and light movie editing (using iMovie) how significant would the upgrade to 4GB be to the user experience? I understand Mac OSX Lion might have some system of saving RAM power si what are peoples opinions - is the upgrade to 4GB worth it? Thanx =]
To address both parts separately
1) The MacBook Air is pretty well sealed up, in addition to the RAM being soldered to the motherboard, the case uses security screws and is pretty tightly packed together. All in, you're not going to be able to upgrade the RAM.
2) I've been using a Macbook Air as a secondary machine for the last several months (13", 1.86 Ghz, 2GB of RAM, 128G SSD) and did not have any problems with RAM running low or slow performance. Used it for the same things that you are considering and overall it worked pretty well. Obviously for any serious video editing, I'd recommend something with more power and a bigger screen, but overall it worked great.
Overall I'd say you're safe getting the 2GB, particularly if you're worried about cost.
If my sources are correct, RAM in MacBook Air is actually soldered to its maindoard, so you won't be able to upgrade your RAM later.
I really don't know if you're going to actually really really need 4 GiB or not, so I won't comment on that, but do keep in mind that you'll have to use that same RAM for as long as you use the computer. I can only say that I'd get the extra RAM if I was planning to use such computer for several years, but then again, my requirements are not your requirements.
It is also interesting to note that many current netbooks have a limit of 2 GiB of RAM an they run fine. On the other hand, I have impression that people tent to use Apple's hardware for a bit longer than hardware of other manufacturers.
In the end, it all depends on how long you're planning to use the computer. At one point in time, you're going to need 4 GiB of RAM, but the question is will you be using your MacBook Air then.
According to Crucial the maximum memory for the Macbook Air is 2GiB and it is non-upgradabe.
It can be neither added to nor removed. It is, as @AndrejaKo says, soldered to the main board.