The quote you're thinking of is:
> "To the well-organized mind, death is but the next great adventure."
This quote is said by Professor McGonagall, not Dumbledore, in the same book. It's part of her conversation with Harry when they are trying to decipher the inscription on the Sorcerer's Stone.
While Dumbledore does have many wise pronouncements about death, like "Of course it is happening inside your head, Harry, but why on earth should that mean it is not real?" in *Harry Potter and the Order of the Phoenix*, he never specifically links death to a well-organized mind.