Did you know that a Google Sitemap (XML Sitemap) and the traditional sitemap are different in many ways?
FACT # 1 - the XML sitemap is made for search engines such as Yahoo, Google and MSN and when viewed by a human, it's not a pretty look because of the layout.
FACT # 2 - as far as the traditional sitemap (HTML Sitemap), it is a graphical way to present your website's layout and when viewed by humans it helps to further aid in their navigation of your site.
FACT # 2 - if you want the search engines to see your webpages, a XML sitemap is the way to go.
Now, you can have a Google sitemap (xml sitemap) and standard sitemap (html) running side by side and it will not negativly impact your site. But a Google sitemap (xml sitemap) is enough to get the search engines to see all your webpages.