Sorry, I know its been covered before but I'm still puzzled.
It seems that the only supported configuration is where the CORE is dual homed - that is, its got two NICs. I'm trying to figure out why - is it because the CORE is trying to serve two roles, one as a media centre, and the other as a firewall?
My preference would be that the CORE does CORE things and boundary protection is left to other things. As it is, if the CORE goes belly-up, none of the other kit in the house would have internet access. I suppose its not hard to unplug the cable that goes into the COREs NIC1 and plug it into the switch that NIC 2 goes to. You just have to figure out who is responsible for DHCP.
Just a thought.