Your xbox is actually downconverting the signal to 480i for your tv to upconvert to it's native resolution. Pretty much all commercial DVDs are encoded at 480p. You are losing half of the picture from the dvd when you go 480i. Going to a 480p player is not introducing an upscaler. A progressive scan DVD player, without upscaling, can output 720x480 (480p - progressive scan) video signals.
