if you have eth0 and eth1 you can specify which card to use by default with the route command (i think thats what your after).
In your case I believe your after a default route for eth0
$ route add default gw my_router_on_eth0_subnet
For example I have two nics on my box and one of them (eth1) is connected to my router on 10.0.0.1, so
this command makes sure anything not destined for a local subnet (other than 10.0.0.X) will go via eth1 which is 10.0.0.10
$ route add default gw 10.0.0.1
Edit for horrendous typo