Now that the nation officially numbers more than 300 million, what next?What will 400 million look like?If demographers are right, we'll hit that mark by 2043. They and other futurists envision a typical American neighborhood that year will be something like this:More than likely it'll be located in the South or West, despite scarce water resources and gas prices that make $3 a gallon look like a bargain. Barely half of the community's residents will be white, and one in four whites will be senior citizens. Nearly one in four people will be Latino and multiracial Americans will be commonplace."We're going to be growing for the next 50 or 100 years, but it's not because of the birthrate," said John Bongaarts, vice president of the Population Council, a non-profit in New York. "If the birthrate were to drop we'd have a very different future ahead. If we were not living longer and had no migrants we wouldn't be growing at all."