Again, as darklordjames is saying, all you'd really be accomplishing with one of these is converting the connection from VGA to hdmi. VGA (480p) is the best you're going to get from the dreamcast. Do you have a VGA connectionon on your tv? If so then your tv will already do what one of these boxes would-- that is convert the dreamcasts 480p output to match the native resolution of your display.
The only way one of these scalers would benefit you would be if: A, they did a better job at scaling than your tv would (which I seriously doubt at those prices); or B, they did the job with less lag time...
However, most of the lag and artifacts generated when processing an image happen during the de-interlacing stage, which you will avoiding by having the dreamcast output a progressive image (480p) via the VGA connection. In other words: there's not much else your display has to do but scale the image, which any decent make/model will do with aplomb and without introducing much, if any, lag.
Of all my dreamcast games only ONE is incompatible with the VGA box: last blade (of course, that's one of my favorite games on the system
). The rest all work even if they don't have the "supports VGA box" tag on the box.