If the site's already being indexed by Google, as John says it is, a sitemap won't really improve its rankings. i.e. Google knows about the pages already, but doesn't consider them important enough to show them in search results (well they're probably there on page 20+).
A sitemap is handy where there are orphan pages, i.e. pages with no links to them from other pages on the site.
We used this guide[/url:2bjhc01t] (written by a former work colleague) for the last website I was involved with (a while back now) and we got onto page 1, sometimes we were number 1. He makes a living out of SEO now.