Upgrade your Macbook Air SSD, reuse the old one as a sleek external drive

Range of prices, depending on capacity & year model of your Air.

http://gizmodo.com/5922781/upgrade-your-macbook-air-ssd-reuse-the-old-one-as-a-sleek-external-drive

http://news.cnet.com/8301-17938_105-57476034-1/owc-announces-ssd-uprades-for-macbook-air/

Or just buy a 2012 Air:

http://osxdaily.com/2012/06/19/macbook-air-2012-ssd-benchmarks-vs-macbook-air-2011/

/ Apple OSX OS X benchmark upgrade SSD external USB 3 case /