iptables and NAT

Hello,

    I want to setup a iptables box on Debian that will act as a firewall
and forward or NAT IP addresses dynamically.  On side A we have an
internal IP scheme with sufficient IP addresses on side b we can call
this the Internet side there are not enough IP addresses to go around so
as users need to connect to side B they get a dynamic NAT address. 
Would shorewall or fwbuilder be best suited for this job. 

Does anyone know of a good how to on this type of scenario?


Jim