Thanks for the reply,
Yes, I am building a firewall, but it also must have the role of a web, mail, ftp, samba server (I am limited to a single machine), so I can't really use IpCop or Smoothwall.
From what I know SME functions as a firewall too, in the 'Server and Gateway' mode, and from what I understand from your post it cannot be made to work with two private networks instead of one. Is this correct?