It is true that it is more difficult to get two identical cards working under Linux but it is not impossible. You should log in as root and use the ifconfig command to determine if the cards have been configured properly. "ifconfig -a" will list all interfaces, whether they are up or not. Do a net search on the /etc/modules.conf file to see how to configure the eth0 and eth1 interfaces for identical cards if the system has not set them up properly.
Telling which card is internal is also tricky when both are the same but it sounds like you have that one figured out as the SME box is assigning DHCP leases, right?
Secondly, your description of how you are connecting to the internet is a bit vague. Are you saying that you connect through a router with a fixed IP which acts as your gateway? You should then enter that as the gateway IP address in your SME config. How is your SME box getting its IP address? The ping command is the best tool to use to test your connectivity throughout your lan.
HTH,
Jim