I have actually done some reading about the 747 tanker. In theory, it's a good idea. BUT, tankers like that are mostly used where firetrucks have trouble getting, like steep canyons. The problem with the 747 is that the airplane is so big, there's no way it can get down low enough to make a difference. The retardant gets too dispersed to actually do alot of good.
It carries 24,000 gallons of water or retardant.