Thanks for posting that. It got me a little bit closer, but I'm still not there. Using your settings, the PCs on the internal network can now obtain an IP address, but they still can't access the internet. The main DCERouter PC can't access the internet either.
Can you also post the output of the following command:
I'm not sure if I need to modify my routing table as well, or if modifying /etc/network/interfaces should be enough.