Not going to help me with the regular tv downstairs. If I get an s-video cable, will that work for both–realizing the HDTV is a one-time deal for now so I don’t care about absolute best picture quality.
Not that it matters to you now, but S-Video is not HDTV. Hooking up a 360 to an HD plasma via S-Video wouldn’t even really be worth dragging the 360 out of your house for. If the situation ever arises again, you’d want to get Xbox 360 component cables, those are the most HDTV friendly of connections (though as XPav mentioned, the plasma may have VGA in).
Since you mentioned S-Video, I have to ask… you do have a 360 Premium, right? Not the core? Because AFAIK the Core ships with composite-only cables, not component/composite like the Premium and if you hook your 360 up to the plasma with composite cables, you’re going to be very disappointed.
$20… yeah, something like that. The cables that come with the Premium already have component though (they have component and composite and you switch between the two with the hd/sd switch on the cable), so if you do have those, you’re all good, those will hook you into the plasma and give good results.