Richest Americans Get Richer: Forbes 400

(Forbes 400, Richest Americans ranking updated for Facebook Founder Zuckerberg charity plants)

NEW YORK (TheStreet) -- The richest Americans are getting richer, according to the latest release of the Forbes ranking of the 400 richest Americans.

At a combined wealth of $1.37 trillion, the 400 richest Americans saw their wealth increase by 8% over last year. The wealth level needed to make the Forbes 400 also pushed back above $1 billion; it had fallen below $1 billion in 2009 for first time since 2006.

Not that the richest Americans don't still have some work left to do in becoming even filthier rich: The 8% increase in 2010 still left the group below the total in 2008 ($1.57 trillion) and 2007 ($1.54 trillion).

No surprise at the top of the list, Bill Gates of Microsoft(MSFT) in the No. 1 spot with a net worth of $54 billion, and Warren Buffett of Berkshire Hathaway(BRK.B) at No. 2 among America's richest, with wealth estimated at $45 billion.

Gates and Buffett have been on a big charitable giving drive this year to convince billionaires to commit a majority of their fortunes to charity.

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Among the Top 10 Richest Americans in the Forbes 400, only Larry Ellison of Oracle(ORCL) and Michael Bloomberg have made the Buffett/Gates charitable pledge. Ellison is No. 3 on the Forbes 400, with an estimated wealth of $27 billion. Bloomberg rounded out the Forbes 400 Top 10 with an estimated wealth of $18 billion.

There are two notable families represented in the Forbes 400 Top Ten that aren't part of the Buffett/Gates group, though they are big charitable donors. The Walton heirs of Wal-Mart(WMT) fame claimed four of the Top 10 spots on the Forbes 400, with a combined wealth of $84 billion. The Koch brothers, Charles and David, of privately held Koch Industries, have a combined wealth of $43 billion and tied for fifth on the Forbes 400.

The most notable of the moves up among the richest was Facebook founder Mark Zuckerberg, whose wealth increased by 245%, the largest of any of the 400 Richest Americans. Zuckerberg was 35th on the list with a wealth estimated at $6.9 billion, with the social media mogul finishing ahead of old media mogul Rupert Murdoch of News Corp.(NWSA), who was at No. 38 with an estimated wealth of $6.2 billion.