I'm not exactly sure what your trying to do but here are all the situations you could be going for
Situation 1
no internet forwarding to lan
Sugestions
cheap
buy a hub to connect to the lan
manually assign ips
slightly more expensive but easier
plug lan into router ignoring wan port
router will suplly ips
Situation 2
Intenet connection sharing
just run the network setup wizard and enable ICS
your computer will assign IPs to the rest of your network through a switch, hub, or crossover cable
Situation 3
Network Bridge
open up network connections highlight the two ethernet cards right click and select bridge
you have to manually assign ips to the lan side but all computer will have access to your modem or router controlling the internet not just the internet signal. Its also useful in different situations for bridging a wireless connection and still giving complete access to the whole lan.
example I have a computer receive wireless connection from router then through hub to xbox or another pc both the pc and the xbox have access to shared files all across the network and the pc can configure the router.
Things to remember you cannot connect connect two computer without a the switch on a router,a normal switch,a hub, or a crossover cable