I think jdubin has the right approach, although a quick search shows that it's not a cheap solution. I found this (http://www.fastvdo.com/SmartCapture/
), which looks like one of the less expensive offerings that has Linux support. If you want to capture something better than composite video, I imagine that the price will go up considerably.
As a disclaimer, I have no relationship with FastVDO and haven't used any of their products, so I can't speak for the quality or their support of them.