I am aspiring to accomplish the same setup you described -- and having the same issues as you earlier described. Would you mind sharing with me the appropriate contents of your (now successfully working) /etc/network/interfaces file, please? Thanks!
In a nutshell I also have a cable internet connection. I wanted to run it into an older Compaq PC with two NICs and Debian Woody to serve as Firewall and Proxy Server. My Debian server would connect in turn to a wireless router.
Rounding out my internal network, the router connects:
Thanks in advance!