how to make a computer to function as a router

[Zhao Peng]

Hi, all linux experts:

I have 2 computers (A & B)

A:
a desktop (running Fedora Core 4),
has 2 ethernet cards. (When I installed Fedora Core 4 on A, I chose "server" among 4 installation types, and I chose DHCP for both ethernet cards.)
One card is connected to a Motorala cable modem via a Cat5 cable.
Another one is an extra one, not used now.
(BTW, I'm using Adelphia rhigh speed internet)

B:
a laptop (running Windows XP professional)

I heard that it's possible to make A a router, so that I connect B to A via a Cat5 cable (not interested in wireless), and then both A and B can get online at the same time

My question is:
How to re-configure those 2 ethernet cards and do other necessary modification?

Any input/clue is greatly appreciated. Specific step-by-step instruction(if I'm lucky) is extremely welcome! :)

Thank you in advance!

Zhao