Setting up a firewall box

[Adrian von Stechow]

After months of denial, I finally admit that my Powerbook G4 500 Mhz
with broken screen (it works for about 30 mins. at under 15 deg
Celsius) is just a piece of junk (even the cd drive is dead). Now i'd
like to set the thing up with a minimal YDL installation and use it as
a firewall between my PC and new PB and my DSL line. Maybe i'll also
use it as a (very basic, unsecure) fileserver.
Any tips or HOWTOs on how to best do this? What's the best solution to
only having 1 LAN port on the PB?

A cool graphical description:
Router -(LAN)- Old PB -(possibly LAN-Switch)- new PB and PC