I have two DVD players that upconvert... Sony DVP-NS77H DVD Player and Sony BDP-S360 Blu-ray Player. Both have excellent reviews about how well they upconvert. Well, I also have the Xbox (the first Xbox, NOT the 360). It doesn't upconvert. So, I just hooked it up to my Samsung HDTV via component and it plays the DVDs at 480i and my TV upcoverts it. Strange as it may seem, the Xbox delivers the best picture quality out of the 3 players I have.
Based on this experiment, I have to wonder if it's really worth paying extra for an upconverting DVD player. As long as the DVD player can send a 480i signal via component, I would think that would be good enough if your TV has a decent upscaler.
So, are upconverting DVD players really necessary if your TV already has a good built-in upscaler?