Maybe James Earl Jones with a Tiamat type echo added to his voice to make it sound more "otherworldly"?
Personally I wouldn't choose JEJ due to overfamiliarity, but maybe, just someone with a good voice who doesn't mind it being tweaked with effects, although if they can great vocal performance out of someone that doesn't need sfx that's fine too.
and yeah, as folk have said, they need to get his personality right.
Pertaining to your earlier question about the book's length being spread over two movies...I haven't read the book in years, but read it over and over agin when I was a kid, so going by that memory bank situation I would say that there is quite a few things in the book that are just a few sentences or paragraphs that take place of a long period of time that could be expanded upon well in the movie.
Like when the dwaves are taken capture in 'barrels out of bondage(?)' and it's said that Bilbo has been sneaking around for months in the place where they are held, trying to figure out a way of escape.
Also, I'd like them to take their time with the travelling through the forest, you need to feel totally hungry during that sequence.
Stuff like the battle of the 3(?)armies at the end too, that's pretty quickly skimmed over in the book but will take up a lot of time onscreen too.
There will have to be lots of dialoge created for scenes that re mainly descriptive in the book, like when the dwarves all congregate at Bilbo's home. So, I mean, from smallish scenes in the book there will be lots of charterisation needed to be added for a movie.
edit: As Kendrall said they are using appendice stuff. But, I don't know how much, at first they said they were going to create a whole movie out of that kind of thing to bridge the LOTR to the Hobbit, but then they said that wasn't the idea they were going with after all.
I think with what needs to be added there's two movies in the Hobbit, better that than rushed through in one.
Really looking forward to it, I only read the 1st LOTR book, I much preferred the Hobbit, my favourite book when I was a kid.