The point of a wikiwiki is to build the web by creating an ever-expanding network of interlinked articles. Don't create links to external pages as part of the article text — more than likely, we'll want to have an article on that subject right here on WWWiki! Links to nonexistent articles are perfectly all right — in fact, they're encouraged (under the appropriate circumstances, of course).
For example, don't write something like this: "Ur-Shulgi was of Assamite clan" — note the external link to another page. Certainly, Assamite.com is an excellent resource for all sorts of information on that clan, but the link is in the wrong place. Where would we put the link to WWWiki's article on the Assamite clan?
Instead, external links should go in a separate section of the article (at the bottom of the page), where these links can be listed for further reading.