What are you trying to do?
If you want to go from VGA to component (YPbPr), then you could use one of those boxes, but some video cards can output that signal already. If you want to use SVideo or composite, you could use one of those boxes but, again, your video card might do it already.
I've used those boxes and have found them clunky at best. IMO, you would be better off spending the same money on a new video card that has the output you need. That way you can simplify your setup and get better quality video (some of those converters are kinda crappy).