yes 6 gig's should be alright for most movies, but it does depend on the movie. remember that a DVD can go up to 18 GB on a single Disk!
if you are concerned about space your should definetly learn how to "frame serve", if you don't already know. it lets you encode a dvd without haveing to use an intermedate file.
I'm sure lots of people can help your with frame serving on this forum.
