I would say the quality jump is somewhere between going from a well recorded CD to a well recorded SACD its there and decently noticable, but not like going from a low quality mp3 to an SACD. Right now it probably isn't worth the jump with that new a receiver, unless you can get a good deal on a newer Onkyo or you have some money burning a hole in your pocket
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edit: Maybe you could see if your dealer would let you test out a new receiver at your home and see if you notice the difference going between DD and TrueHD on the same flick, while not a perfect test it is a decent enough one. Another option if they won't give you a loaner to audition is just do it in a show room with their speakers and see if you notice the difference.