technically the cheapest way might be to add a second NIC to the first computer and use a crossover to the 2nd computer.
the BEST (and not so expensive way) is to by a router ($50-$100 prob).
then you go cable -> cable modem -> router -> computers
make sure to buy a router that has at least 3 ports 1 for the cable modem, 2 for you. most liekly this means buy a 4 port router, which is fine.
you coudl by a 2 port router and then tack on a hub or switch behind it and connect the computers to the hub.
4 port router is the easiest adn its easy to setup and most have the added protection of being a firewall which will help you.