Updated from 3:42 a.m. EDT

DoubleClick

( DCLK), a New York-based online advertiser, announced Monday that it would acquire

@plan.inc

(APLN)

, a Web market research company, in a cash and stock deal valued at $120 million.

The acquisition of @plan, which seeks to sharpen its clients' online marketing and advertising strategies, allows DoubleClick to form a research division, an arm that could give advertisers even more intelligence about the habits of Web consumers.

Stamford, Conn.-based @plan provides clients with clues about lifestyle issues, product preferences and demographic data. Last month, for example, the company released results of a survey gauging consumers' opinions on

Napster

and other online song-swapping services.

The deal would give the combined company a client base of more than 6,000 advertising agencies, advertisers and Web publishers, and the opportunity for an even wider reach, said Kevin Ryan, DoubleClick's chief executive, in a statement. The companies hope to finalize the agreement in the fourth quarter of this year.

Dana Serman, an analyst at

Lazard Freres & Co.

, compared @plan to

ACNielsen

( ART), a market research firm in the offline world that offers insights into consumer attitudes, opting for depth over mere statistics.

Research on viewers traveling back and forth from one television station to another is always valuable to advertisers, who are waging an increasingly competitive battle for the attention of consumers. ACNielsen, for instance, provides data on radio and television ratings.

"But in the online world, it's even more critical," because of the sheer number of Web sites, said Serman, who rates DoubleClick stock a buy. His firm has not done any underwriting for the company.

The acquisition of @plan fulfills a goal set earlier this year. Greg Ellis, DoubleClick's vice president and general manager of research, had joined the company as the chief engineer of a plan to build a research operation.

"As we took a look at the market and the needs of media buying and e-commerce communities, it became clear that @plan was a strong partner," said Ellis, who will assume the role of @plan president after the companies formally unite.

Explaining the pact, DoubleClick said it would exchange $9.25 for each share of @plan stock -- 80% in DoubleClick stock and the rest in cash. As a result, the deal represents a 28% premium for @plan shareholders, based on @plan's closing price Friday of $7.25.

The terms of the transaction, though, could change if DoubleClick's stock price during a period of 10 trading days ending four trading days before @plan's shareholder meeting falls below the $23.87 mark, according to the companies.

DoubleClick finished Monday regular trading down $1.75, or 5%, at $35.75, meanwhile @plan closed up $1.50, or 21% at $8.75.

The combined company will have its headquarters in New York, but @plan, which lists

eBay

,

Microsoft

and

CNN Interactive

, among many others, on its client roster, will continue to operate in Connecticut.

Meanwhile, Mark Wright, chairman and chief executive of @plan, will remain in his spot until the merger is completed, and then assist with the transition as a consultant.

DoubleClick added fuel to a debate over Internet privacy last year when it announced its intentions to merge anonymous data it collects online with a database on catalog shopping habits put together by an acquired company,

Abacus Direct

. The company, under pressure from critics, later postponed those plans.