i don't think you will notice much differance if any. just make sure you use the component cables and you have the switch on the cable switched to hd. (if that is the setup you are using)
What is the Xbox outputting? If you have it set to 1080i, is there any chance it may be upconverting a 480i DVD to 1080i?
I doubt it becuase so far most upconverting DVD players use a DVI output.