Video resizing

I have some clips that I need to edit then burn to DVD and most of them are 640x480. I have already converted most of them to either DV or QT AIC at 640x480 and have started editing them, but I just realized that they will need to have bigger dimensions as they will be played full screen on a TV. Which option would be best for video quality?
1. Go back and re-convert all of them with the proper dimensions
2. Keep editing and when I export them from iMovie set the proper dimensions
3. Export them at 640x480, then when they are played expand it to fit the screen.

Thanks

 

Some are 640x480 and some are 720x480, though when I set the output for the 720x480 to the same dimensions it looks stretched so I set it to 640x480 and it looks normal.
NTSC
I would like to keep it 4:3 so it doesn't look stretched and have letterbox on the left and right sides.