128 GB is not enough space! I just got my MacBook Air a few months ago. The new one is out with a spiffy backlit keyboard and 256GB SSD, but I'm not ready to buy a new one yet.
It was so much easier than replacing the hard drive in a white iBook! Piece of cake!
The screws are very very tiny! Keep them in a dish or a cup or stick them to tape. Don't lose them! And don't let the cat help.