2012 Macbook Air. Is upgrading RAM worth it?

Hi,

So I've had a 2008 aluminum unibody Macbook running on 2GB RAM, 2GHz Core2Duo for a long time. For while I ran OS X Lion but after a hard drive failure it's been restored to Leopard (not Snow Leopard) and things still run pretty zippy even with my outdated system.

I'm considering upgrading my computer to a 2012 Macbook Air, but I'd like to keep it for as long as I can. My question is whether or not it would be worth spending an extra $100 to upgrade the 4GB RAM to 8GB RAM since I read that this can't be done by yourself anymore.

Thanks.