The current Rev B Air comes with either 120GB HDD or 128GB SSD; are you buying the earlier Rev A model? Apple recommends
9GB of free disk space to install Leopard. For this relatively small sized processor 1.6/1.8 you ideally want to be looking at SSD which will improve the UI experience quite noticeably. I have the 1.8 SSD Rev B model which very quickly became the most frequently used Mac in the house and due to its light weight accompanies me to work every day along with O2's mobile dongle. The Air's screen is better than the Macbook's, but I was surprised to find the built-in iSight camera is lower res which makes it extremely grainy in all but the brightest of light. In all honesty, I'm completely enamoured with the Air and will probably buy the Rev C after it. The Air finally brought some definition to the way I use my laptops, for the first time a truly portable Mac.