Dune is the only essential book in the entire series, as Frank Herbert (as well as his son Brian, writing with Kevin J. Anderson) makes his entire postulate in that volume. Having said this, if you enjoy the original, keep going until you feel it's no longer entertaining. I read the originals years ago before he died and felt increasing disappointment with each successive book. I picked them up again 30 years later and didn't enjoy the original as much as I'd remembered, but found the sequels much better than before.
At this time, I read all the Dune books written by Brian and Kevin also. While they have a much different writing style than Frank, it's not always a bad thing. On the down side, Miles Teg went from being an amazing general (as written by Frank), allowing him to plan for otherwise-unforeseen circumstances that allowed him and Duncan to escape, to a completely incompetent ship's security chief (as written by "the others"), which allowed crew members to be repeatedly killed and replaced by Face Dancers while he cluelessly tried to figure it out. On the up side, Frank often bogs down the narrative with endless internalization that doesn't progress the plot or add anything new, just beats a dead horse until he's tired of belaboring the point, while "the others" keep a more active pace in the writing.
This being said, I wouldn't recommend a first-time reader to start chronologically (at The Butlerian Jihad at this point, counting only novels), although I enjoyed the Legends and Great Schools books immensely. I really enjoyed reading about the events that made the universe the way it came to be in the time of Dune. Also, two of my favorite Dune-universe characters came out of these books: Vorian Atreides and Norma Cenva.
If you've made it this far, and enjoy Dune itself, I would continue with the original series through Chapterhouse: Dune, then continue with the next two books chronologically (Hunters of Dune and Sandworms of Dune), which gets you to the end, then I'd start with The Butlerian Jihad and continue chronologically from there.
Those are just my two cents.