Making a Linux Router/firewall/proxy from a Dell Poweredge 1950 - need help

I posted this in another forum as well because I'm in desperate need of help with this. I've been stuck at the same point for some time now..

Ok, so what I want to do is make a router/firewall/proxy (maybe add webserver/FTP as well). Just to start off I want to say that I have moderate knowledge of Linux, enough to administer it from the CL. I have setup routers before but it was years ago and I've forgotten some of the details involved. What I do is a base LAMP install, with DNS, Samba, DHCP server, OpenSSH and then Webmin for easier administration. I've also installed EHCP (easy hosting control panel) in the past but have not at this point.

So, what I want to know is how do I setup the NIC's in the etc/network/interfaces file. Let's say that eth0 connects to the modem and eth1 & 2 are internal adapters. Currently my network is running a Linksys WRT54GL with DD-WRT and the router is set to DHCP for the WAN connection and DHCP is running on the internal network as well. The modem is at 192.168.254.254 and is giving the router an address of 192.168.254.1 my internal network is 192.168.1.1 (192.168.1.0/24). I would like to setup my internal router address to 192.168.1.1 so I guess I need to set it to static in my interfaces config and then set my eth0 to dhcp. Does this sound correct?

So if I do the above my only question is how do I setup the routing tables after that? I always get messed up when I need to make the switch from my Linksys router to my Linux box. I'm not worried about firewall rules at first I can change those once I have the router up and running. I just don't know if I need to make some kind of bridge to bridge the eth0 and eth1 (external NIC and internal NIC).

Can someone help me get past this troubling hurdle?

I have used shorewall firewall with success in the past. Shorewall is a wrapper for iptables. It can intergrate with webmin so that you can configure it through there.
You might also want to look at some linux distros dedicated to routing like open wrt.
I am not clear on exactly what you are trying to achieve and there are probably several ways you could configure it. I would start by disabling dhcp on any devices that you do not want to be utilising dhcp.

You want to make sure you use a different subnet on each interface.

http://www.clearfoundation.com/


All in one, simple to use and setup, free and exactly what you are looking for.

https://help.ubuntu.com/community/Router


or http://www.smoothwall.org/