I've had similar setups, and the problem with sharing the cable modem this way is that the computer connected to the cable modem directly must always be on in order for the other computer to access the internet.
This can be a real pain, but if you're living to live with it, it can be done as Larry suggested.
Personally I switched to a router/hub combo because I couldn't stand having to turn the other computer on and wait for it to boot up.
Here in Canada, the appropriate router with a built-in switching hub is less than $200 cdn., so if you're in the states, it should be significantly cheaper.
If you're ever doing work involving both computers - for example transferring large files between the two of them - I will disagree with Larry in saying that a 100BT hub can be faster than a crossover cable. Using crossover, your max thoroughput is 10BT (or 20BT effective full-duplex), while using a 100BT hub, even a half-duplex one, will give you 100BT connections (assuming both cards are 10/100BT cards).