> So far I've set up a PII 400MHz  system with 128MB ram and a 128MB CF
> card.
> It's doing fine and the memory use sits at only 19%, but there hasn't been
> much stress on that machine so far.
> Only a max. of 4 users now and no traffic shaping or so.

Depending on your needs,....EBAY !

You can troll for a GX150 or GX240 small form factor for less than
$100 US. I buy them all the time and your going to get an
awesome/reliable pc that is probably between 700Mhz- 1.4Ghz

You can also pick-up used intel / 3com nics too, as these typically
don't push processing off onto the pc processor. Read the FreeBsd HAL
for what works, or look at http://wiki.m0n0.ch/
just my 2 cents

~don