The question of whether to use www or not is known as a "canonical" issue. I'm not sure of the etymology of the term, but it means that if both versions are available on the web, the search engines will index both versions, resulting in "duplicate content" which means that they see two identical pages.
In the resulting SERP (Search Engine Results Page), the very last thing that the search engines want is to return two or more links to pages that are identical because their users would leave and try a different search engine, so they push down one or both versions of the page to at least page 3 of their SERP.
The answer is to use a 301 (permanent) redirect on one version to send visitors (and search engines) to just a single version of the page. For example, if you want to use www, then the 301 would send any visitor who forgot to type in the www immediately to the www version.
The 301 is a server header code that is available in the htaccess file, so you should contact your hosting company and have them make the change. Google allows you to set the canonical version in the Tools section of their Webmaster Tools interface, but you should decide beforehand which version you want to use and then make certain that you stick with that convention throughout your site, including the sitemap.xml file.
Do not use a 302 (temporary) redirect - it should be a 301 (permanent) redirect.
You should do this before the search engines can see any of this, and once everything is straightened out, you can re-submit your site to Google, Yahoo, and MSN to "kick-start" their robots.
The choice of whether or not to use www is entirely up to you, but your decision will show up in the final SERP. It may be easier for your hosting company to set up the 301 redirect to go to the www version, but the choice is entirely up to you.
So, decide which version you want to use (www or not) and then:
1. Have your hosting provider set up a 301 redirect in the htaccess file to send visitors to one place or another.
2. Fix the sitemap.xml file to match the 301 version (www or not).
3. Add a line at the beginning of the robot.txt file to tell the world where to find that file ( Sitemap: http://www.example.com/sitemap.xml ).
4. Set the canonical preference to match in Google's Webmaster Tools.
5. Re-submit your site to all three major search engines (the smaller ones should eventually find everything).
Also, you can manually edit the sitemap.xml file to include a missing file and let the search engines know which canonical version to use, as in:
<url>
<loc>http://www.example.com/</loc>
<changefreq>weekly</changefreq>
<priority>1</priority>
</url>
<url>
<loc>http://www.example.com/clients.html</loc> <changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
Labels: SEM, SEO