A scan converter would have quality comparable to a video card's TV-out, of course it depends on which one you get and what you are comparing it to. One advantage is that many built-in TV-outs limit your resolution to 800x600, while many scan converters let you use 1024x768. The quality is no better, but less fooling around changing settings, etc.
But I suspect with Divx any kind of TV-out would be plenty good.