DVI is digital, which means the signal isn't deteriorated by digital-to-analog conversion as opposed to VGA. Most, if not all projectors at least support DVI input. I think DVI makes the most difference on flat panels, like LCDs and plasma screens.
S-Video and RCA don't support HDTV resolutions, IIRC.
Disclaimer: I haven't used HDTV in any form ('cept for the WMV HD clips), so don't take my comments as absolute truth
