After doing some reading, it looks like ALL signals can be upconverted to 1080i. Assuming I'm correct, that's nice as it means all games would play at a higher resolution.
One thing I realized, though, is that my widescreen television would automatically stretch out a fullscreen (4x3 aspect ratio) image to 16x9 if I use component cables once the HDTV is told to pump out a 1080i resolution. For most fullscreen-only games, the video image would look greatly distorted. Usually when I play fullscreen-only games I like to turn on my HDTV's "Just" aspect ratio, which stretches out the corners to fill the entire screen, but leaves the middle section essentially untouched so that distortion is minimalized. The "Just" aspect ratio is unavailable when component cables are used and the HDTV is given a 480p or 1080i image. ("Just" is only an option for the typical 480i resolution.)
So I want the 1080i resolution image, but I also want to play the games in the "Just" aspect ratio. Ultimately that means I would have to export the signal from the upconverter to the HDTV using an S-Video cable.
Questions:
[1] Which would produce the greater image quality?
a 480p image connected directly from my Xbox to my HDTV using component cables,
or a 480p image connected from my Xbox to an upconverter using component cables which would send the signal out at 1080i using an S-Video?
[2] Is there even an upconverter that will allow a signal to be outputted using S-Video?
[3] Is there an upconverter that allows for multiples inputs? All I have run across are upconverters that will only let you use one input. I have an Xbox, Playstation 2, and Gamecube, so I'd like to be able to have them all upconverted without having to manually switch their system cables.