Most directories will only help you increase PR and SERPs. However, there are some niche directories that get search engine traffic and some of those clicks make it back to you. Take a look at the directories in your niche then try a google search for some of the long tail terms on their page and see if they come up.
http://www.allydirectory.com/blog/2008/04/06/how-to-choose-a-quality-web-directory/