The extra warranty is rather pricey, but its probably best to take it.
Also with osx, you're going to want at least 8 gigs of ram, with the air you cant upgrade the ram after you've bought it.
At least 8 gigs of RAM is a good bet regardless of OS. That said, I know many people happily chugging away on 4 gig airs.
And it's not just the Air - most (all?) Macs can't have RAM upgraded these days. And increasingly not just Macs now either - it's become a tradeoff for the compact size and clean lines, easy to make since the vast majority never upgrade their RAM anyway...