Question: is 1.8 ghz better than 1.6?
Answer: It's a big advantage if you use it mostly in very quiet environments. If not, then I'd go for the 1.6 HD model. I think the 1.8ghz, although with its clock more than 10% higher, isn't necessary. The Macbook Air is a very snappier machine, but it won't be good for any cpu intensive jobs like video or audio editing.