You can use CANOPUS PRO CODER V2.0 or SORENSON SQUEEZE to convert the files, or you can export directly from your video editor.
Pixel Wars: Because pixel aspect ratio 0.9 (3:2) is intended for TV, when viewed on a computer monitor the video can look 'fatter' than viewed on tv, due to pixel aspect. So, when downscaled to 320x240 par1.0 software will correct automatically the aspect to make it look in the proportions as they look on a TV, so, don't worry about losing quality or the images to shrink if you downscale, because your video file (720x480 0.9) is not displayed in full resolution, you are actually viewing a fat video, because computer compensate the aspect ratio. sometimes, when i downscale videoclips, the files looks better than original file. This is because the pixel aspect is the correct for monitors.
Go on, downscale your files, and dont worry about anything... well... actually yes: the codec you will use to encode your clip.