A billion dollars just doesn't go as far as it used to. For the first time, it takes more than $1 billion to earn a spot on Forbes magazine's list of the 400 richest Americans. The minimum net worth for inclusion in this year's rankings released Thursday was $1.3 billion, up $300 million from last year.

The new threshold meant 82 of America's billionaires didn't make the cut.

Collectively, the people who made the rankings released Thursday are worth $1.54 trillion, compared with $1.25 trillion last year.

The very top of the list was unchanged: Microsoft Corp. founder Bill Gates led the list for the 14th straight year, this time with a net worth estimated at $59 billion. He was followed by Warren Buffett of Berkshire Hathaway Inc. in second place with an estimated $52 billion and casino mogul Sheldon Adelson, No. 3 with an estimated worth of $28 billion.

Larry Ellison of Oracle Corp. maintained his ranking at No. 4, with an estimated net worth of $26 billion.

But the list showed some notable changes.

Joining the top 10 of the country's richest for the first time were Google Inc. founders Sergey Brin and Larry Page, who tied for fifth place. The 34-year-old moguls' wealth has quadrupled since 2004 to an estimated $18.5 billion this year, while their company's stock value has surged 500 percent.

And, lower down, almost half of the 45 newcomers made their millions in hedge funds and private equity investments. The youngest member of this year's list was 33-year-old hedge fund manager John Arnold, who joined the ranks at No. 317 and a net worth of $1.5 billion.

"Wall Street really led the charge this year," said Matthew Miller, editor of the Forbes list. "God only knows if they'll be on it next year. It really just depends on what the market does."

Surging oil prices also helped some members of the list. Oil baron brothers Charles and David Koch also broke into the top 10, sharing the No. 9 spot with estimated wealth of $17 billion. Their ascension bumped the Walton family, heirs to the Wal-Mart Stores Inc. fortune, from the top 10 for the first time since 1989.

The discount retailer, struggling with a slowing economy and higher gasoline prices as well as merchandising mishaps, has seen its sales lag behind rivals like Target Corp.

Climbing 19 rungs to No. 7 was casino tycoon Kirk Kerkorian, who doubled his net worth to an estimated $18 billion. The 90-year-old investor is a majority shareholder in MGM Mirage — operator of the MGM Grand, Bellagio and other casinos — which saw record profits at several of its Las Vegas hotel-casinos.

Rounding out the top 10 was Michael Dell of computer maker Dell Inc., who was No. 8 with an estimated $17.2 billion.

The magazine confirmed the worth of an individual's holdings in public companies by using the Aug. 31 closing stock price, and estimated the value of private companies by evaluating comparable public firms in the industry. The list also takes into account philanthropic donations.