A sluggish economy? Not for the two-thirds of the wealthiest people in the U.S. who boosted their average net worth by $400 million to a record $4.2 billion. The top guy: Microsoft founder Bill Gates, with $66 billion. Sergey Brin and Larry Page have a net worth of $20.3 billion each. Mark Zuckerberg is on the list at $9.4 billion.