Hi.
I have a white MacBook (I think late 2008 or early 2009).
It's the last white plastic model with irda port before the unibody white plastic.
I've upgraded to 4GB ram (max is 6 by two 4GB sticks but those are pricey).
The OEM hard drive is very slow. I've upgraded to WD blue, WD black, and then to SSD. Each HD upgrade made big speed gains.
Latest OS is Lion.
More RAM and SSD will also extend battery life.
Sure Air weighs less. All depends on your budget. If its $400 you may get one with a SSD and use the old drive as external.
I think the original Air is too slow.
If you run Snow you can get Apple remote and watch your iTunes stuff via front row. Install Perian too.
Good luck with your hunt...