Confirmed that they won't wait for the last book(s):
"I guess we spend more time worrying than the fans," Benioff said. The two of them visited Martin at his home in Santa Fe back in February to pump him for information about where all the stories are going, so they can properly set them up in the world of the show, and "It was incredibly useful," according to Benioff.
"There's no question that this will be better for us if the books come out before the various seasons come out," Benioff added. "That said, we're not going to take a two-year hiatus (to wait for a book). The little kids are growing older, the show's got momentum now, and the show must go on. We're just hopeful that it will all time out."
Interview with the producers from http://www.hitfix.com/whats-alan-watching/game-of-thrones-producers-say-season-3-as-big-as-were-going-to-get