I can see advantages using the core as a gateway - even with only one networkcard. Besides security consideration: it is far more easy to split it physically up, if you`ll ever have the need for more network speeed.
On the other hand having only one networkcard made it really difficult. - Now that I know I can`t believe how blind I`ve been, but noone mentioned anything about the DSL-Modem/Router`s routing ... and even if it does work with my Fritzbox it doesn`t work with a friends Zyxel.
--> I would suggest to make a seperat entry for having one networkcard, like there is one for the two http://wiki.linuxmce.org/index.php/Why_dual_network_cards%3F