I could be wrong, but 480i should have a lower framerate than 720p, because it's interlaced rather than progressive, which means the image won't be refreshed as often. But I'm a complete n00b when it comes to these sort of things so I could very well be wrong.

Anyway, I think what GooseGaws said is true. It doesn't matter on what resolution you play, as the game is still rendered at the same resolution.

 You == correct.  NTSC framerate (that is, 480i framerate) is spec'd to be just under 30FPS (i think its like 29.99 or something like that, Wiki probably knows but I am too lazy to look it up).

 I think he was asking more for any future purchases though, unless I misread him. If he gets an HDTV (generaly 60+fps), then he is wondering if making it only output at 480p will make the framerates better in games where that is a problem.  In that case, 

^^this wins

Well, it may win, but essentially, it would be much like answering a true or false question as "true, unless it's false"

I guess it's progress though. I'd appriciate it if anyone with a PS3 could actually go around and do some digging on these. Just find a scene that does have framerate issues at 720 - 1080 and then drop it down to 480 and see if there's any difference.