Also, I'm not sure if anyone's said this explicitly, but you can add files to your iTunes library without duplicating or moving the existing file. Uncheck 'Copy files to iTunes Music folder when adding to library' (in 'Preferences' --> 'Advanced') and you can use the 'File' --> 'Add to library...' command to make a compatible file accessible via iTunes without having to copy it to iTunes' library.
You can just drag 'n' drop the file onto your iTunes movie list, and it'll add it to the list and create (if necessary) and iTunes-friendly version, without messing with the original.
When I first started ripping my DVD collection to my Mac, I nearly jammed my HDD because I didn't realise that there were two versions of each movie once I dragged the rip over to iTunes. Now I know to delete the original and empty the trash when I'm done.