Yes, you need a network interface card (ethernet card). However, most cable companies will give you one when you sign up. If not, they are like 10 to 20 bucks brand new. You can use a 10BaseT card or a 10/100BaseTX card. Neither will be faster than the other over a cable modem.
The cable company may also provide you with a modem, although they may charge you a little extra for that. We use the one the company gave us, but we might buy our own soon.