XSi was never meant to record video, and thus you get really strange things. Premiere should let you make a new sequence with 21fps, 768x512 progressive, you just need to make a custom sequence. That said, if you really care about video at all, just get an actual HDSLR rather than using that program.
Your other option is to use something like avisynth and apply frame doubling to get 24p, 25 or 29.97 fps, either as a avs (with avs importer plugin) or by re-encoding using avs and megui.
I don't hate macs or OSX, I hate people and statements that portray them as better than anything else. Macs are A solution, not THE solution. Get a good desktop i7 with Windows 7 and come tell me that sucks for photo or video editing.
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