Look at Skyrim!
Skyrim will have 1 million final preorders in NA on the Xbox 360 alone, and roughly 1.5 million combined. That could mean sales of 2.5 million first week worldwide and 8-9 million lifetime!!!
Skyrim is going to get Red Dead Redemption/Assassin's Creed level of sales and lift Elder Scrolls to the mega-franchise category.