I agree with those who mentioned Ryan Gosling as Jake/George. In my opinion, he's one of the best actors out there today.
As for Sadie, maybe someone like Jessica Chastain. (Loved her in "the Help") She is a redhead, but that could easily be changed for the role.
I think Rachel McAdams would be great too....although would be awkward for Ryan and Rachel.
I like the idea of Jim Parsons as Oswald. Would be interesting to see him in a dramatic role. I enjoy Jim on "Big Bang", he really carries that show.