* Fed ramps up stimulus to support economic growth * Fed move boosts risk appetite * Treasury prices fall sharply after brief bounce By Wanfeng Zhou NEW YORK, Dec 12 (Reuters) - U.S. stocks rose and the euro rallied against the dollar on Wednesday after the Federal Reserve ramped up its monetary stimulus as contentious U.S. budget talks heightened uncertainty about the economic outlook. Treasury prices briefly added to gains, before falling after the U.S. central bank committed to monthly purchases of $45 billion in Treasuries on top of the $40 billion per month in mortgage-backed bonds it started buying in September. In a surprise move, the Fed also adopted numerical thresholds for policy, a step that had not been expected until early next year. "The $45 billion number confirms what the market was looking for. It's additional (quantitative easing), which should be risk-positive," said Brad Bechtel, managing director at Faros Trading in Stamford, Connecticut. "It underpins the equity market and, to me, is a nice framework for a risk rally that I would expect to carry over into the first quarter. The fiscal cliff is obviously a concern but if we get through that, it should be risk-positive." The Dow Jones industrial average gained 40.81 points, or 0.31 percent, to 13,289.25. The Standard & Poor's 500 Index rose 6.20 points, or 0.43 percent, to 1,434.04. The Nasdaq Composite Index added 4.71 points, or 0.16 percent, to 3,027.01. The MSCI global stock index advanced 0.5 percent to 338.61 points. The euro rose 0.6 percent to $1.3075, after hitting a session peak of $1.3079 after the Fed announcement. The dollar rose 0.7 percent to 83.06 yen. The euro had jumped sharply minutes before the Fed announcement after Silvio Berlusconi said he would withdraw as a candidate in Italy's coming election if outgoing Prime Minister Mario Monti ran as the head of a "moderate" coalition. The benchmark 10-year U.S. Treasury note was down 5/32 in price, with the yield at 1.673 percent.