Well, PAL and NTSC are analog modulations. After you capture PAL or NTSC video into your Mac, they are no longer PAL or NTSC - they are digital. They may have the same resolution and frame rate as an analog PAL or NTSC video, but they can still be converted into either one (with some quality loss - but this is true of analog conversion too). There are no NTSC DVDs, there are only DVDs that have 29.97fps, 720x480 MPEG-2 video to maximize video quality on an NTSC analog output. So just master your video at 29.97fps, 720x480, and it will automatically work on NTSC DVD players. ffmpegX can probably help you here, and Toast might be able to do it as well.