Here is what I suggest you do:

(1) Shutdown. Poweroff. Take out one of the network cards.

(2) Reboot. After reboot, execute the dmesg command and copy the name of the device driver for the ethernet card (or cut and paste the info).

(2a)If you can't find the ethernet card in the dmesg output, note that as well.

(3) Shutdown. Poweroff. Switch cards.

(4) Repeat step (2) for the second card.

(5) For both cards, while they are out of the box, look to see if they have on-board jumpers.

(6) Post the results to this thread and we will help you with the next step.