Changing Sitemapdatasource (1 Viewer)

I would like to change the SiteMapDataSource and the Menu to limit user access to specific pages. I have created a textbox on my Web Page with a User Role. Based on the UserRole, I would like to display either the full menu or a limited menu.

Any suggestions on how and where to do this?

http://msdn2.microsoft.com/en-us/ms178431.aspx
http://msdn2.microsoft.com/en-us/library/system.web.ui.webcontrols.menu(VS.80).aspx (several links about halfway down the page)