I heard about the series a lot. Got dragged to the first movie by my then-10-yr-old eldest nephew, who was big on the books. After seeing the movie (twice, mind you), I decided I should catch-up on the books to prepare for the inevitable sequels. Read through them twice so far.
The movies, like MOST movies based on books, cut a fair bit out and change a little bit. For instance, the first film leaves out the "bottle trap" that Hermione and Harry have to go through to finally get to Quirrel. In the second film, it's McGonnagal who tells of the story behind the Chamber of Secrets instead of the ghostly History of Magic professor in the book. You get the gist.
As for movie 7.2,...if you've read through the books by the time it comes out, there's really no reason to see ANY of the movies. But, if you must see one to prep for it, see Deathly Hallows 1. Somehow, I don't see them having a "Previously on 'Harry Potter'" montage at the start of 7.2.