If I understand you, it sounds like you have a ADSL modem
Is that correct?
You have two computers, one with two ethernet cards, where 1 is connected to the ADSL modem, the second ethernet card connects to
to my good comp with a crossover cable.
If I understand your configuration, the connection between the comp with two ethernet cards and your "good comp" needs to be a separate sub-net. This can be any private class A class B or class C private sub-net. For class C 192.168.x.x will do. If I were you, define teh IP addresses for the two interfaces on teh direct cable. Onnce done, try pinging. That will work if they are in the same sub-net. Next you have to get the private sub-net to talk to the outside world. You need a routing function.
I have never done this kind of setup, but lots of others have. I think you need to setup iptables. Have a look around the board, and in the tutorials.
The easiest way is to buy a router and plug all your boxes into it; but that costs a few bucks... I have seen four port routers for as little as $49.00. Have a look around.