Except for the Kasper and the German VGA box, all the other ones are merely frame doublers.
You will -NOT- see much of an improvement. In actuallity I find the SVHS output superior. I feed both SVHS and VGA to my 19" LCD flat panel.
The VGA looks "fuzzy" because of the frame doubling (actually quadrupling on my LCD's resolution) whereas the SVHS looks good, but like an overly anti-aliased picture.
The Kasper boxes actually force the Xbox into 480p mode and above so it looks -MUCH- better.