can i add or change the HD capacity after purchased?

Part of your answer: http://www.ifixit.com/Guide/MacBook-Air-Models-A1237-and-A1304-Hard-Drive-Replac ement/860/1

Not sure what model you have, but the idea is sound.

SSD prices and types and manufacturers vary greatly. Anywhere from $150 - $800 for reasonable quality.

Macsales has some nice videos on how-to do it http://eshop.macsales.com/installvideos/macbook_air_2011/

Plus as a reknown seller round here check their prices as well.

Stefan

Just keep in mind that there "may" be a warranty issue. While the SSD in the Air is easily replaced, Apple has not officially indicated that this is a replaceable part. So far as Apple is concerned, there are no user serviceable parts in the MacBook Air. This is sort of a gray area as I have yet to see any comments from anyone needing to have their machine serviced under warranty who has also replaced the SSD. If anything, keep the original SSD around and put that back in if you need to have it serviced (if for no other reason, they will sometimes replace the drive during their service procedure and you might not get the same one back).

Again, I'm not saying it "will" void your warranty... it's just something to keep in mind until we get a few reports from people who have replaced their SSD and had warranty service done.