Another option (if you're in a real hurry) is to stretch the output of your original comp to 720x480 by adjusting the stretch parameters in the Output Module settings.

The correct way to do this kind of work though is to create your animation in the first place at the square-pixel equivalent size of the NTSC frame. Note that that size is actually 720x540 rather than 640x480 (the larger size means no upscaling is involved in the conversion).

If you want to know more about this kind of thing, here's a great article: <http://library.creativecow.net/articles/gerard_rick/pixel_madness.php>

Best of luck with it.