If you want to capture a legacy, analog video signal, then you need a TV card or a graphics card with video-in feature.
FireWire is used for input of a digital data stream from a digital video camera.
Either is easily and cheaply added using a PCI card ... although both are better placed on a higher speed bus. So chipset integrated FireWire as seen on the latest SiS offerings, and TV-in on the AGP graphics card, are to be preferred.
regards, Peter