JupiterResearch predicts online sales growth will double to $18.9 billion by 2010, up from $9.3 billion at the end of 2004. Search will be the key driver, overtaking online banner advertising.

In its annual online advertising forecast, JupiterResearch said web search ads are set to grow faster than 12 percent per annum over the next five years while banner displays should grow only around seven percent.

According to the report, growth in search-based ads reflects both rising advertiser confidence in the market, as well as the sophistication of software technology that interweaves advertising into many of the most popular ways that people search the internet.

"There is phenomenal momentum behind search engine advertising," JupiterResearch analyst Gary Stein said in a statement. "The number of advertisers using search to market products continues to grow, as does the overall efficiency of the market -- search engines are getting even better at making money off search engine results pages," he said.

But search engine marketing is only one element fueling the growth of overall online advertising, JupiterResearch said. Classified web advertising is expected to grow at nearly 10 percent, reaching $4.1 billion in 2010, it forecast.

Another growth area is rich media, spawned by the growing number of households with broadband. Rich media spending may grow at a 25 percent compound annual growth rate, reaching $3.5 billion in sales, and streaming media will grow at a 30 percent compound annual growth rate, to $943 million, by 2010, JupiterResearch said.