640 by 420 is actualy a good Video resolution...
You cant make a film that is at the same res as the game you your windows.
Its actualy very close to TV resolution 720x540 (actualy its 720x 486 with rextangular pixels)

So in order to compress a film into a size that most machines can play and download (talking MB's here) 640x420 is pretty,,, good
MPG or DIVX or Quiktime with Sorenson3 will Probbaly give ya the best results when compressing your edited film. so Even If you put it on a dvd to watch on your tv you would make it 720x486 at 30fps.,, or actually 29.99fps
MonkeyMeat