If the final game keeps the same overall shape of the current playtest documents, I'm in for the core books, at least. Besides that, as a psionics fan, I'll wait for the Psionics Handbook (whatever they call it) and will probably purchase it as well.
Anything after those will depend on the kind of product Wizards intends to make. I'm in for good setting and adventure stuff, as well as material that expands on the flavor of core races and classes, with new rules only as needed. If we end up in a new age of "Complete Everythings" full of new classes, feats, races, subclasses and other similar stuff, Wizards won't be seeing a lot of my money after initial release.
Cheers,