I've always felt that what was published about the Realms is a good place to start.
There is a lot of info out there about certain places, but not so much about most places (I'm not counting anything 3e and beyond). The other thing I like is that you can just set your adventure before or after the published material...or explain how that Volo guy got it wrong.
I'm excited to see Oerth get some props though.
I've had some GH books that I picked up at a used bookstore years ago (b/c they were like a buck or two each
) that I've always thought would be cool backgrounds.
If I work on the Realms, and you work on Oerth and we can convince Steve McDonald to get active again so he can finish his Dragonlance campaign, we will have a pretty good representation of the AD&D 'published' universe.