so if ur adsl comes in on eth00 and ur LAN is connected to eth01, set eth00 to the ip subnet that the router wants to use, set ur eth01 to ur LAN subnet, then under NAT enable it on eth00 and u should have connectivity. u may need to set a static route to the modem.
i have a router behind my main to separate a small problem subnet and i had to static route from them to the main gateway. the gateway is 192.168.0.1, their network is 192.168.1.0 so i set
Destination Netmask Type Metric Gateway Interface
0.0.0.0 0.0.0.0 Net 1 192.168.0.1 Up
hope this helps