Google stock broke through the $400 price level for the first time Thursday amid optimism about a surge in online advertising during the holiday season and continuing projections for rapid, long-term growth.

The search engine that went public at $85 a share in August 2004 has continually exceeded Wall Street forecasts for profits. At the new level above $400 – shares closed at $403.45 on Nasdaq – Google’s lofty stock market value of $117.7 billion makes the Silicon Valley firm worth tens of billions of dollars more than the combined value of Internet titans Amazon.com and Yahoo.

Wall Street analysts announced new price targets for Google stock as high as $450 a share. The company’s ability to generate billions of dollars in cash, in addition to rapid search results, is an underlying factor that explains the extraordinary rise in its stock price.

But company officials have warned that a slowdown in its growth rate is inevitable as the search engine grows larger.