1) It all depends what you're going to be using it for - If you plan on any serious use then it's worth getting 4Gb. If all you're doing is a bit of surfing and email then 2Gb is plenty.
2) If you were buying a PC, would you wait for Windows 7? There's no release date for Snow Leopard yet so if I was looking at buying a Mac now, I'd not even be taking Snow Leopard into consideration.