Updates with market close

By Noel Randewich

Aug 27 (Reuters) - The S&P 500 notched a record high close on Wednesday ahead of quarterly results from Nvidia, Wall Street's most closely watched event this week, which will test the rally that has pushed valuations of AI-related companies to levels that some investors view as too high.

Shares of Nvidia, the world's most valuable company and the leading supplier of cutting-edge AI processors, bounced between gains and losses ahead of the report, due after the market closes.

With Nvidia making up about 8% of the S&P 500, its financial results affect vast numbers of Americans who use index investment funds to save for retirement.

"Nvidia is going to produce humongous revenue gains over the next nine months, on top of an already humongous revenue base," said Jed Ellerbroek, portfolio manager at Argent Capital.

"Investors should prepare themselves for a world where Nvidia is a double-digit percentage of the S&P 500."

Shares in tech and AI heavyweights Alphabet GOOGL.O and Amazon AMZN.O, Microsoft MSFT.O and Meta Platforms META.O, among Nvidia's biggest customers, were mixed.

Enthusiasm for companies related to AI has fueled big gains in technology stocks. The S&P 500 now trades at over 22 times expected earnings, its highest price-to-earnings ratio in four years, according to LSEG.

Concerns about the pace of the AI rally increased last week after OpenAI CEO Sam Altman warned of a potential AI bubble.

Nvidia options implied a roughly 6% swing for the shares in either direction following the results.

Investors will closely watch the impact of the Sino-U.S. trade war on Nvidia's China business earlier this year, along with the effect on forecasts from its recent revenue-sharing deal with the U.S. government.

According to preliminary data, the S&P 500 .SPX gained 15.35 points, or 0.24%, to end at 6,481.29 points, while the Nasdaq Composite .IXIC gained 43.49 points, or 0.20%, to 21,587.76. The Dow Jones Industrial Average .DJI rose 143.76 points, or 0.32%, to 45,561.83.

Investors were also watching for developments related to U.S. President Donald Trump's attempt to fire Federal Reserve Governor Lisa Cook, a move likely to face legal challenges.

If Trump succeeds, he would nominate a replacement to the central bank's board who could be expected to back his policy preferences, challenging the central bank's independence.

Investors are pricing in a 25-basis-point interest-rate cut in September, according to data compiled by LSEG, with most big brokerages also leaning in that direction.

New York Federal Reserve Bank President John Williams said on CNBC it is likely interest rates can fall at some point but policymakers need to see upcoming economic data to decide if a rate cut is appropriate at the Fed's September meeting.

MongoDB MDB.O soared after the software-maker raised its annual profit forecast.

J.M. Smucker SJM.N fell after the Jif peanut butter maker missed first-quarter profit estimates.

(Reporting by Johann M Cherian, Sanchayaita Roy and Purvi Agarwal in Bengaluru, and by Noel Randewich in San Francisco; Editing by Pooja Desai and David Gregorio)

((johann.mcherian@thomsonreuters.com;))