Rowling's Holocaust denial aside, here's kind of a rundown of the info you'd get in the book:
I've not seen the movie to compare, but in the book, Dumbledore is already near certain Voldemort has made Horcruxes. The diary's Tom Riddle phantom is just too complete and too whole to be a memory or what-have-you, but the way Voldemort treated the diary made Dumbledore suspicious. A Horcrux is supposed to be your safeguard against dying, but Voldemort treated it as a kind of weapon. He left it with Lucius with the eventual goal of reopening the Chamber (though he "died" before that could happen). There were no safeguards, no protections, he didn't even tell Lucius what it actually was.
Based on how casually he treated the diary, Dumbledore theorized that Voldemort didn't stop at that one. Slughorn's memory isn't important because it told Dumbledore that Voldemort had made a horcrux, it was important Voldemort because asks about the possibility of making multiple ones, thus confirming the theory (though I suppose the ring had already safely proved it).
The most important piece of information offers, however, is the number. When Voldemort asks about it, he says, "
Wouldn't it be better, make you stronger, to have your soul in more pieces, I mean, for instance, isn't seven the most powerfully magical number, wouldn't seven--?
"
That tells Dumbledore that he didn't stop at two or even three, he made six of them (the seventh piece being in his own body). That gives him and Harry a concrete amount of horcruxes that have to be dealt with, so they can know when the job is finished. Looking at the scene on Youtube, I think the fact he made
seven
of them is what is he's talking about when he says it's "beyond anything [he] imagined."
I also took a look at the orphanage scene and it seems they omit the reason Dumbledore knows about the seaside cavern. In the book, there's a scene where Dumbledore speaks with the matron (who drinks a shitload of gin) and she talks a bit about how fucked up the kid is even though they never catch him doing anything. One of the incidents was when they took the children to a day at the beach. Tom lured two other kids into that cave and did something so fucked up to them that they refuse to speak of it. Dumbledore had already found the ring at his Merope's home, so he started investigating other places that Voldemort had a connection to and presumably he felt a tingle at the cave.
(Incidentally, they really butchered the orphanage scene. Little Tom Riddle is far more volatile and excited about being special in the book whereas he's just a cold little Omen kid in the movie).
One other change I noticed in the scene I watched is that Dumbledore says the horcruxes could be anything. While that's true, he actually refutes this in the book when Harry says it, stating that Voldemort would only want to put pieces of his soul into important objects. Which brings me to Hepzibah Smith; since her wiki article only shows an illustration, I assume she's not in the movie either.
There's another set of memories in the book from when Voldemort was working at Borgin and Burkes, an antique shop that had a reputation for dealing in dark magic artifacts. As part of his job, he frequently met with Hepzibah Smith, a collector who claimed to be descended from Helga Hufflepuff. After he butters her up, she shows him two of her prized artifacts: Salazar Slytherin's locket and Helga Hufflepuff's cup.
Shockingly
, she's poisoned two days later, supposedly by her house elf mixing up sugar and the poison.
Based on that and Voldemort's love of Hogwarts, Dumbledore's pretty sure he had the goal of turning the Founders' artifacts into horcruxes. Obviously he has something from Slytherin and Hufflepuff. He doesn't know if Voldemort found something of Ravenclaw's, but the only Gryffindor artifact is the sword Harry found in the second book which is sitting in Dumbledore's office.
Finally, Dumbledore talks about Nagini and how she's weirdly intelligent and connected to Voldemort, so he surmises that she's his latest and final horcrux.
So Dumbledore actually has a pretty solid idea of what Voldemort's horcruxes are.
-
The Diary (destroyed)
-
The Ring (destroyed)
-
The Locket
-
The Cup
-
Something of Ravenclaw's
-
Nagini
I think that's pretty much everything from the book. Anyway, fuck Rowling.