If the DVD you are backing up is a Single Sided and Single Layers DVD (called a DVD5), then using DVD Decrypter to write then burn tan ISO of it will result in an "exact copy" of the original.
If its duel layer DVD (a DVD9) then you will need an ecoder/transoder to reduce its size down to fit to a 4.7gig DVDR blank. Currently, DVD2One will convert the movie only to fit to a DVDR, while Instant Copy 7 and DVD95Copy are capable of reducing the whole dvd (Menus, Movie, etxtras, etc) to fit to a DVDR. All with different amounts of quality loss. DVDXCopy can backup a DVD9 to fit to 2 DVDR's with no quality loss. You can also do a conversion of the movie only and substantial retaining most of the original quality using DVD2SVCD with CCE. (see the DVD Tutorial section on this one).
All have their pros and cons, some should be called Beta versions, some being fixed almost daily. Some do as they say and some have a long history of success.
Suggest you have a read of the number of tutorials/URL's/threads posted through the forum on the use of the above programs and the Tutorial section.