I've just got a question, and I need it answered BEFORE I begin with extensive work.
I've got ADSL at home with a 4 ports modem/router (supports NAT). Behind that router I
want to place 1 PC, with 2 NIC's. I want THAT PC (with SMOOTHWALL.org) installed to serve
and protect my entire network. (with NAT)
Furthermore I want to run an FTP server on my LAN, that can be accessed from the outside.
I also want to run a VPN server, and IRCbot and a IPtables firewall on my PC (2 NIC's).

How can I do this the fastest? I heard someone mention bridging, but that can be done by
IPtables directly, I think.
Can the DHCPserver on the modem/router give away IPadresses on my LAN or do I need my
PC (2 NIC's) to do that? Because it will be kind of a double NAT situation.

What's the best way to do all of this? (especially with gateway IP's etcetera)

Smoothwall does all the functions you are asking for...
No need for bridging anywhere.
Let it do the dhcp for the LAN.
It has a DMZ zone for public accessible servers... Easy config interface too..

The dsl modem/router could be bridged to present the external number to the Smoothie box, but your ISP will need to arrange that.
It's certainly not necessary.
NAT at the router if you only have 1 ip number off the dsl...