have no clue what smoothwall supports, check smoothwalll website or run smoothwall and check the old fashioned way
if the DMZ will be using dhcp to assign IP's to you personal machines all you would need to do is use dhcpcd eth0 or whatever the hardware name of your ethernet card is on each personal machine----after they are setup with the ethernet card is actually working with a kernel module loaded for it
if the DMZ will be using static ip's then each personal machine would need a static ip-----use ifconfig eth0 <ip> <netmask> up
also add the DNS #'s to /etc/resolv.conf
have no freakin clue what GUI crap ubuntu has to do this, but im sure it can be done that way as well
as far as your DMZ server, have no clue what smoothwall can and cant do, but iwconfig is usually used for wireless networking, and it will depend on how your wireless network is setup, it will need ip forwarding and NAT running though, as well as some at the very least very basic iptables rules