When you use n.overridden you call and merge the original macro.
So this:
<override_macro name="sidebar_widget" requires="node_page,servlet">
<n.overridden />
<n.ads_widget />
</override_macro>
Is equal to:
<override_macro name="sidebar_widget" requires="node_page,servlet">
<n.actions_widget/>
<n.people_widget/>
<n.subscribe_widget/>
<n.categories_widget/>
<n.archives_widget/>
<n.ads_widget />
</override_macro>
You can change the order replacing the n.overridden with the original macro items in the order you want.