NEW YORK (FORTUNE) - In the late '90s, bubble investors paid almost anything for outfits that promised astronomical growth. Millionaires were made and fortunes lost on stocks hyped up, not on fundamentals like earnings growth and margin expansion, but rather more ambiguous measurements like price to sales and eyeballs.

Investors have since returned to more comprehensive analysis of business models and financials. But there are still some stocks that are commanding astronomical valuations.

The companies below may have great potential, but their P/Es - based on expected 2006 or 2007 earnings - make us nervous.

Up in the stratosphere is Audible (Research). The company, which pioneered downloadable audio books, is expected to earn 2 cents a share this year and while the stock is off 23 percent in the past 12 months, it still trades at a P/E of more than 460. Listen up for first quarter earnings, which will be announced in early May.

Rambus (Research) is a chip-tech firm, which develops and licenses interface technologies. Betting on advantageous outcomes of litigation currently in process, investors have sent the stock up 170 percent since January, giving it a P/E of 160. But earnings reported on April 19th were less than stellar, with higher revenues primarily driven by royalty fees and earnings down due to inclusion of non-cash stock-based compensation.

The online merchant of software for salespeople, Salesforce.com (Research), certainly has sold investors on its business model, but at 150 its P/E is a little harder to buy. The stock has more than doubled in the past year and Morgan Stanley's analyst calls the valuation "rich," putting a $31 price target on the stock.

Investors love data-storage and high-speed-access company Equinix (Research), even though it's not expected to make money this year. The stock is up 50 percent over the last year and its P/E is 112, based on estimated per-share earnings of 51 cents - in 2007. The stock has enjoyed quite a run. First-quarter results, set to be reported on April 26th should offer some idea whether the pace will continue.

At the low end of the high end is Linux developer, Red Hat (Research). In early April, Red Hat bought open-source software maker JBoss, giving the stock a boost, but it has since slipped a bit on concerns of competition from Oracle and others after Larry Ellison told the Financial Times that Oracle may develop its own Linux system and has considered buying Novell. But despite the talk, Red Hat is still up over 175 percent since last April and its P/E has climbed to 73.

Find out which seven stocks and three funds FORTUNE says are buys.