There is no upgrade once you've received your order from Apple. Get all the RAM you can afford now. You'll get more money back when you re-sell it years from now anyway, especially since MBA's with lots of RAM will be fairly rare.
I know that the ram is solder to the motherboard. Would it be possible to upgrade the RAM to 4gb. It uses the same graphics as the SR macbook, correct? So it should be able to accept 4gb, right?