Ripsnort, each computer needs a NIC (network interface card - network card). You may also need a router, it depends on how many IP addresses you get with your cable service. (If you only get 1, you need a router. If you get 2 or more, you are ok, though a router can still be useful for adding more computers later. In some circumstances you can also reduce you monthly bill by dropping to one IP.)
I'd recommend Linksys products myself. Us Robotics makes a cheap router that seems to work ok as well.