If you've got an old doorstop 386/486 hanging around there are several free microdistributions of UNIX/Linux that run off a stiffy (unpack themselves into 8MB RAM) and replace expensive dedicated routers/firewalls.
Absolutely, MAC
They also seem to be more reliable/predictable than the so-called software one built into Windoze products.
I'm playing with FREESCO (
http://www.freesco.org/ ) right now on an old 486 box, it looks really neat and easy to configure.
Watch this space...
I will, with interest. I'm currently protected by
www.smoothwall.org 