Which is why I said run separate component cable from your DVD player to the projector. S-video will only give you 480i.
S-video will be fine for now for the Xbox and PS2 since having the switching through the receiver for these is available. With only one device using the component video input to the projector all you have to do is select that input on the projector when you want to watch a movie.
Another cheap alternative is a component video switcher. Here's an example:
http://www.inday.com/rgb4x/rgb4x.htm
(If you already spent the money for the HD adapter for the Xbox. They always charge more for these damn adapters!)