> if NICs in it to do NAT/Firewall/proxy work.

You can get really gutsy and put a caching name server and a small web
cache on it, too...that'll help with bandwidth utilization as well.

> 
> I have a follow-up for this: Does PC hardware (in general) require
> a keyboard and monitor to function?  I remember pulling an 8Mhz
> 80186 XT clone from the apartment trash one day about 6 years ago,
> and seeing the message on the screen "No keyboard found, press F2
> to continue."  Despite the obvious lack of design ability by the
> people who put a message like that in the BIOS, is that true of
> all (or even most) of the cheap older machines I might find?

I think every PC I've seen has an option to disable testing for the
keyboard and you can compile NetBSD to spit its console messages out a
serial port as well as provide a login there.  So you don't need a monitor
and keyboard on the box once you get it running.

> 
> Remember, the specific desire here is for a machine to fit on the
> top shelf of a small linen closet. This will require something
> small like a mini-tower or pizza-box sized thing at the largest.
> Low power would also be a very good thing, as there will be very
> little air circulation in there.

Mine here at home runs on a sparcstation 2 using the onboard AUI connector
as well as an Sbus LE card.  I think I have a total of $125 in the box.