I'm using 2 ethernet cards in my system. No wireless. I don't know if that would cause a problem.
However, when I pull up Advanced/Network Settings via web admin page
EXTERNAL_IFACE is eth0. This has to be the case for the core system.
EXTERNAL_IP 192.168.1.105 (in my case)
Domain=LinuxMCE Computer name=DCERouter
DHCP server on Core:
X Enable DHCP server
then some ip address range stuff
Number of network adapters: 2
Your core has the following network adapters:
1.External network card eth0
X Obtain IP address from DHCP
2. Internal network card eth1
IP address: 192 168 80 1
Subnet mask 255.255.255.0
I hope this gives you something to compare to.