Sorry - I need to make this clear:
Most upconverting DVD players are cheap, not expensive. A really good upconverting DVD player, that outputs the native resolution of your projector, plasma, or other display will give you better results over either component OR HDMI/DVI than just a decent progressive scan player. But, in my opinion, still not worth the cash you must lay out for it.
Most good upconverting DVD players are upwards of $500.00 and a dedicated home theater PC (the best way to do it) may easily run $1,000.00. Likewise, really good DVD player will quickly run $1,000.00 or more. It just isn't worth it in my opinion. I am very happy with 'really good' instead of best. Best to me isn't dropping $1,000.00 on a good DVD player. It is waiting patiently for a year and buying a HD disc player and then having far superior quality, color, and sound than DVD can currently provide even with the best upconverters.
But, that's just my opinion and my thoughts on the matter.