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.