2 Ethernet Cards

Hi,

my server was working fine with ONE ethernet card

I just add a second card
Installation seems to be OK
IP address on a different network (but nothing connected yet)

Now, I lost all my connection !!!!
There is no Firewall at the moment (No Iptables...)

In fact, if I open Mozilla, I cannot connect to internet.
If I disable the second card (eth1) -> Go back on line !!!

How, can I make sure that any application will use eth0 and NOT eth1 ???
How the selection is done ?
(Because if Mozilla is selecting eth1, that will, obviously, never work
out :-(
.