Just to clarify things
www DOT isaserver DOT org/tutorials/isa-server-2006-network-templates.html
Overall you need to use the Server Publishing feature and the Network Rule feature as well.
blogs DOT technet DOT com/b/isablog/archive/2008/06/24/server-publishing-with-isa-server-2004-2006-and-route-relationship-between-networks.aspx