Hi Vinny,
if you want the server to do firewalling as well, then you'll need 2 nics, and also if you want to install something like squidgaurd or dansgardian, if you make it in gateway mode people can't just bypass the proxy server, a few handy hints after you install it (takes 15 min's for a first timer) you will do some little configuration on the machine it's self then detach the monitor and keyboard insted connect to it via
http://servername/server-manageryou can only connect from an internal location, under remote access enable ssh locally, and just putty to it in the futur or configure things via the server manger.
it was my first experience with linux using sme, and it took me no time to set up.
most contribs are located here
http://mirror.contribs.org/smeserver/contribs/and you'll find plenty of howto's as well.
also one other thing if your planning on using 2 nics use different ones, some poeple some times have problems if they are identical.
hope that helps
cheers