The first of the two films will likely fill in a lot of back story taken from The Silmarilion and Lost Tales, with the last 45 minutes-hour of the movie being the first part of The Hobbit. The second will probably conclude The Hobbit and fill in some of the story between the end of The Hobbit and the start of Fellowship. I suspect a lot of that "extra" material will come from the War of the Ring series.