Updated from 9:35 a.m. EST

Google's ( GOOG) march on $400 is complete.

When last month's third-quarter earnings blowout prompted some analysts to predict the stock would top $400 , skeptics were quick to emerge. After all, Google shares already looked gaudy, fetching $303 apiece heading into the company's Oct. 20 financial report.

But now Google is up more than 350% since its public offering last year and the stock opened at $401.68 Thursday. Many investors clearly expect the stock to continue its upward trajectory. Volume in out-of-the-money Google calls is rising, indicating that people expect the stock to keep jumping.

Lately, the shares were at $402.13, up $3.98, or 1%.

Google has reached a point in its history where merely mentioning that it's considering adding a new service is enough to create a buzz. Newspapers are concerned about the impact that the company's Google Base service will have on their classified business. Publishers have filed suit to block Google's plans to scan books.

There are some kinks in the Google juggernaut. It hasn't been able to make any headway in markets beyond search, including e-mail and comparison shopping, and it continues to face heightened competition from rivals including Yahoo! ( YHOO) and Microsoft ( MSFT).

"It's warranted to be cautious," says Larry Haverty of Gabelli & Co., which owns a small position in Google among its $28 billion in assets under management. "They have attracted a lot of attention from competitors."

Even so, figuring out the Mountain View, Calif., company's growth prospects over the long term remains tricky.

"There is no question a lot of analysts think that Google is going to rule the world," said Darren Chervitz, director of research at Jacob Asset Management, which owns Google shares. But he cautions that "it is a business that is entirely dependent on advertising. It won't take much of a slowdown for people to question the multiple."

Chervitz, whose firm also owns shares of the TheStreet.com Inc. ( TSCM), the publisher of this Web site, doesn't believe Google's business is showing any signs of faltering. Even so, his apprehension highlights the challenge Google poses to investors who survived the Internet bubble of the late 1990s.

The numbers are mind-boggling. Google came public in August 2004 at $85 a share. Its market valuation now tops $100 billion, and its shares trade at a price-to-earnings ratio that's about five times that of the S&P 500.

Slow and Steady
Google's relentless rally

On the other hand, the company is growing much faster than any other outfit of its ilk. Google's net revenue, excluding so-called traffic-acquisition costs, is expected to grow 93% from a year ago in the fourth quarter to $1.26 billion, according to analysts surveyed by Thomson Financial.

By contrast, Yahoo! is projected to see a 36% gain in sales, while revenue at Microsoft is expected to jump 11%.

Some investors warn against putting too much faith in those sorts of comparisons, though. It's hard "to accurately forecast immediate and long-term operating performance," says Les Satlow, who helps manage $320 million in assets under management for Cabot Money Management in Salem , Mass., including Google shares. "What you can do is make what you consider reasonable assumptions."

By Satlow's reckoning, Google could be a $450-to-$500 stock if it "really fires on all cylinders for the next five to 10 years." Conservative estimates, meaning 25% to 30% operating-income growth, could mean that the shares are fairly valued at current prices, he says.

To some investors, such as Paul Meeks of NMH Advisors of Charleston, S.C., Google shares are expensive now. Using what he calls "very aggressive" assumptions over the next five years, he has calculated a price target of $322. He follows the tech sector but doesn't own the shares because he focuses on small-cap stocks.

Meeks says his calculation "assumes that Google continues to grow like a weed even when it's much larger, never sees its outlandish profit margin pressured by competition, and is able to earn as much on its future assets as it does now," says Meeks, who doesn't currently own the shares. "Although in the short and probably intermediate term, GOOG is indeed special and deserves a premium, this is ridiculous."