U.S. stocks tacked on further gains to push the S&P 500 to a two-year high on Monday, continuing a steady upward march investors believe will continue in 2011.

Volume was light, as is expected through the rest of 2010. Investors stayed with sectors that have led a sharp December rally, including financials, energy and materials stocks. The S&P 500 is up 5.7 percent for the month and 11.8 percent for the year.

We have seen this year-end rally refuse to give up any ground, said Bucky Hellwig, senior vice president at BB&T Wealth Management in Birmingham, Alabama.

The combination of improving economic data, additional stimulus from the Federal Reserve and the extension of tax cuts was keeping equities in demand, Hellwig said.

The financial sector <.GSPF>, which has lagged the broader market, was up 0.4 percent as some investors bet the sector could be among the leaders next year after a strong December.

However, American Express tumbled 3.4 percent to $42.50 after Stifel Nicolaus downgraded the stock to hold from buy.

Boeing Co also kept gains in check. The stock was down 2.7 percent to $63.27 after the Seattle Times newspaper said a delivery delay could be announced for the plane maker's 787 Dreamliner aircraft.

The Dow Jones industrial average <.DJI> dipped 13.78 points, or 0.12 percent, to 11,478.13. The Standard & Poor's 500 Index <.SPX> added 3.17 points, or 0.25 percent, to 1,247.08. The Nasdaq Composite Index <.IXIC> rose 6.59 points, or 0.25 percent, to 2,649.56.

U.S. markets will be closed on Friday to observe the Christmas Day holiday on Saturday.

Energy shares led the way up as the price of oil rose nearly 1 percent in choppy trading. Chevron added 0.4 percent to $88.88.

Amazon.com helped boost the Nasdaq on optimism the company will benefit from improved holiday spending. According to research firm comScore, U.S. online sales are up 12 percent to $27.5 billion so far this season compared with a year ago.

Shares of Amazon gained 3.2 percent to $183.29, while the S&P consumer discretionary sector <.GSPD> was up 0.6 percent.

Amex shares are down 7.8 percent since last Thursday when the Federal Reserve proposed limiting debit interchange fees at 12 cents per transaction.

Stifel said the company is increasingly exposed as a cut in fees will significantly increase pressure on higher-cost payments alternatives like Amex.

Nick Kalivas, senior equity index analyst at MF Global in Chicago, said some of the optimism over the financial sector was sapped by last week's proposal.

There was the feeling there could be some opportunities in the banks and I think the interchange (proposal) cooled that, said Kalivas.

Food and beverage company Sara Lee Corp has been in talks to sell itself to Brazilian meat producer JBS , but the two companies are at odds over price, a source said. Sara Lee shares rose 2.5 percent to $17.69.

About 6.37 billion shares traded on the New York Stock Exchange, the American Stock Exchange and Nasdaq, well below last year's estimated daily average of 9.65 billion.

Declining stocks slightly outnumbered advancing ones on the NYSE by 1,534 to 1,482, while on the Nasdaq, decliners beat advancers 1,383 to 1,273.

(Reporting by Leah Schnurr; Editing by Kenneth Barry)

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