1080i would use a resolution of 1920x1080 AFAIK. Doesn't your TV support 720p (1280x720), which I imagine would be the better choice for gaming on a PC, since it's progressive (uses the full refresh rate of your TV)?
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
