The sitemap submission is only one piece of the puzzle in the overall process of marketing your website. On its own it will not help to improve your rankings with Google.

However, if you are engaging in link building strategies on the web to increase the critical mass about your website in the Internet community, one of your items on your "To Do" list would be to submit your website xml sitemap to the search engines.

All three of them (Google, Yahoo, MSN) have a process to submit your sitemap, which ultimately insures that they index all of your pages correctly. It also helps to speed the process up of indexing along with – at least with Google – provide you critical information about their perspective of your website. This will obviously help you in the optimization process too. Google has a Webmaster Central area that discusses this further.

http://www.google.com/webmasters/

Also take a look at the Sitemaps.org website which all three major search engines agreed upon in 2005. General info.

http://www.sitemaps.org/

Lastly, if you are interested in using a Sitemap generator to help you automated the creation and in multiple formats (xml, ror, html, text) take a look at this:

http://www.smartfindsmarketing.com/xml_sitemaps.php

Hope this helps.
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