Last month went to pubcon seo conference in Boston.

One of the topics that was discussed by googleguy was the using of underscores in urls.
ex:www.mysite.com/new_topic.html which the keyword for the url would be " new_topic " instead of " new topic "
So it is better to uses dashes instead ex:www.mysite.com/new-topic.html which will give you the "new topic " as the keyword for url