* Investors seek clarity on fiscal negotiations
* Banking, energy stocks may see biggest reactions to news
* Final read on Q3 GDP, weekly jobless claims on tap
* Futures: Dow up 1 pt, S&P up 0.6 pt, Nasdaq up 4 pts
NEW YORK, Dec 20 (Reuters) - U.S. stock index futures were flat Thursday amid uncertainty over U.S. fiscal negotiations, as President Barack Obama threatened to veto a controversial Republican plan, suggesting a deal wouldn't come as soon as many investors had hoped.
* Equities have been volatile this week, though they notched strong gains Monday and Tuesday as the latest offers from President Barack Obama and Republican Speaker of the House John Boehner indicated progress was being made in negotiations.
* On Wednesday, however, shares sharply fell as a rise in tensions in budget negotiations threatened to unravel recent progress. While many investors continue to expect a deal to avert the so-called fiscal cliff, steep tax hikes and spending cuts due to take effect in 2013 that may tip the economy into recession, a deal may not come as quickly as had been previously anticipated.
* Boehner, in a brief press conference Wednesday, said his chamber would pass a proposal that Obama had already threatened to veto, which spares many wealthy Americans from tax hikes needed to balance the budget. Obama has already agreed to reductions in benefits for senior citizens.
* Stocks have rallied recently on any sign of progress in the negotiations, led by banking and energy shares, which tend to outperform in times of economic expansion.
* S&P 500 futures rose 0.6 point and were above fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures added 1 point and Nasdaq 100 futures rose 4 points.
* Stocks ended a two-day rally with Wednesday's decline, though they remain near a two-month high.
* IntercontinentalExchange Inc is in talks to buy NYSE Euronext, the operator of the New York Stock Exchange, according to a source familiar with the situation on Wednesday, in a multibillion dollar deal.
* Google Inc agreed to sell set-top TV box maker Motorola Home to Arris Group Inc for $2.35 billion in cash and stock, the companies said on Wednesday.
* Investors await the latest economic data, including weekly jobless claims and the final estimate on third-quarter gross domestic product, both due at 8:30 a.m. ET (1330 GMT) The pace of growth is likely to be revised up to 2.8 percent from 2.7 percent, while claims are expected to rise to 357,000 in the latest week, from 343,000.
* November existing home sales are due at 10:00 a.m. and are seen rising 1.3 percent, compared with 2.1 percent in October, while the Philadelphia Federal Reserve Bank's December business activity survey is forecast to come in at -3.0 compared to -10.7 in November.