I'm reading this book now [ and I haven't finished, so please NO SPOILERS ], and I've naturally cast Garcia as Astorre, and Ed Harris as Cilke. I'm not a fan of Depp as Astorre really, but I like the idea of Norton as Cilke as someone mentioned. I also like Laura San Giacomo as Nicole. I think she has the right look.
I also think that Gwyneth Paltrow could be Rosie - she has a very loving look, but can also be quite cold.
De Nero/Pacino - great for the Don - I would prefer Pacino.