There are also IDE interface compatible flash disks which are enough
to store a small distribution able to behave as a firewall.

Last time I looked, flash disks were expensive. I would want to do this with
parts I could find lying around for free. The main problem is losing
outgoing/incoming mail when there is a power cut. The flash disk would be
ideal, just too expensive. (i.e. more than a fiver).

Christian.