Alright I em unfamiliar with networking. I know quite a bit but not enough to do what I wish attemp.

I am looking for any websites, tutorial, articles or even a great explanation on how to achieve the following:
A computer acting as a server with either unix or linux installed, 2 network cards, one recieves from modem the other pass' the traffic to the router which then pass' to each computer.

I have everything I need I just em not sure how to do this.

Any help?