use latest version of freebsd and set up ipfilter.
lots of tutorials on the net regarding this.
i have a p100 mhz with 82 meg ram running 2 100mb PCI ethernet cards and i notice no lag whatsoever, and we're on a very high speed lan.
just install minimal software (basicly cvsup, pine, bash, and lynx)
with the correct ruleset you can make it very hard to connect to from the outside network let alone hack. and with only ssh allowed to connect on the inside interface its really secure from external threats.
then lock down the filesystem (setting ro access is a good way to do this)
i run tripwire for filesystem security and keep mad logs of everything.

try it, it works really well.