If you have good internal navigation and strong backlinks for the site, then a sitemap won't have much of a positive effect. A sitemap can help if you have an awkward URL structure, but it is not a panacea.
For instance, let's say you've a three page website with the following keyword "plan": Index Page - Optimized for Quick Brown Fox, Page 2 - Optimized for Furry Pink Elephants, and Page 3 - Optimized for Purple People Eater. The anchor text on the sitemap for the links to those pages should be as follows:
The Ultimate Quick Brown Fox Website Furry Pink Elephants - Our Vision of the Future Purple People Eater - Our Fears for the Future