A recent study by Engine Ready, an Internet marketing company, analyzed 18.7 million visits over two years to Web sites run by 27 of the company's roughly 500 clients. The study found that paid listings had a slight edge over "organic," or unpaid, search results: visitors who clicked on paid links were 17 percent more likely to buy something, and they spent about 18 percent more on each order.

But neither of these is the most valuable kind of Web site visitor. That honor goes to the people who arrive at a site by typing its Web address directly into their browsers or clicking on a bookmark. Such visitors, who tend to be repeat customers, linger the longest, spend the most money, and are the most likely to "convert" to buyers, doing so on 3.3 percent of their visits. On average, their visits are worth $5.69 apiece.