I would use Google traffic estimator to find keyphrases. Otherwise your might get a keyphrase that doesn't get enough people. In other words, if only 10 or so people are searching for a keyphrase then those people probably won't go to your site. Your site is
in competition with 9 other sites on the front page. In addition, you might have to compete with pay per click ads.
I don't really agree with the poster's message. I think single keyphrases can be good in some instances.
For example, if your using adsense or something similar then single keyphrases are fine. All you want people to do is click on a pay per click ad or search box. Your not interested in real buyers. Of course, the people who made the ad are interested in such people. However, they have many ads spread out all over the net. Therefore, they make thier money by the end of the day. That's assuming thier pay per click campaign is successful.
On the other hand, if your selling a product then probably you don't have ads strung across a million sites. Therefore, the audience coming into your single website is very valuable. In that case, you want to target long tail keyphrases.