Networks Coyote Router in DMZ Port forward

I have now been scratching my head over this for quite a while and im not 100% on where this should be. So i put it in networking because it is a networky thing. But its on linux so i dont know how good its going to be. Oh well. I just set up a coyote linux router to replace the portforwarding in my linksys because its knackered really! I am now left with a very complicated mix of what to do.
Coyote wants 2 nics as standard, one going to the internet so connected straight to an adsl modem or whatever. The seccond going into a hub for the internal lan. But i have the 4 port switch built into my router that i use. So its impossible to put it between the lan and the modem. So i put the coyote box in the dmz so any requests for my public ip on the internet get straight through to the coyote box. Then it should be a simple case of putting another ip on the other nic for internal redirect traffic. BUT when both nics have the same subnet and are connected to the same network i get a load of "Neighbour table overflow" messages. Which i am guessing is because the coyote router is trying to take all traffic local or not and stick it wherever it feels like but it always comes back to itself.
So i need to find how to isolate traffic going from one card from going back to the other.
I am trying having the external nic as a 10.0.0.0 address and internal as a 192.168 address but that seems to be giving me the same problem.
what a bint.
Ben