I'm looking for a computer + os + application software that has the
following properties:

- inexpensive (sub $500)
- quick on/off (think PDA)
- very very quiet
- can run on a desktop or a laptop
- runs only a few applications -- web browser, email client
- os can be anything

Ideally this computer can run without the need of a backend server.
However, I would consider that approach too, which would be more like
an Xterminal.

Any suggestions or pointers are much appreciated.