The 400 richest Americans, including U.S. citizens now living abroad, as compiled by Forbes magazine. Following each name is the person's age were available, net worth, hometown and source of wealth. Where more than one name is listed under a number, there is a tie. 1. Gates, William Henry III, 49, $51 billion, Medina, Wash. 2. Buffett, Warren Edward, 75, $40 billion, Omaha, Neb., Berkshire Hathaway 3. Allen, Paul Gardner, 52, $22.5 billion, Seattle, Microsoft, investments 4. Dell, Michael, 40, $18 billion, Austin, Texas, Dell 5. Ellison, Lawrence Joseph, 61, $17 billion, Silicon Valley, Calif., Oracle 6. Walton, Christy, 50,...