If you added new namespaces, it would increase the memory requirement somewhat, but not by much. Unless you added hundreds of new namespaces, or if you already have hundreds of sub-forums, you were probably already close to your memory limit before adding the extra namespaces.
According to the error message you posted, just one of your plugins (products have lots of plugins put together) has just under 50,000 lines of code. That is a red flag. I would try to track down the plugin that is that long and improve it or remove it. By comparison, most VaultWiki plugins are between 4 and 10 lines long.
EDIT: Some more info...
If your vB version is 4.2.0, class_core.php ~ line 4623 is the hook template_render_output, so if you have any products using that hook, you might want to start looking there. If your vB version is < 4.2.0, it's possible the line is the actual rendering of the template output, in which case, you would be looking for very long templates that are likely to be used on forum.php.
Either way, if you added multiple namespaces and they all have hundreds of thousands of articles in them, that might have an effect on performance if all the namespaces appear together in the forum list on forum.php. In this case, you might want to hide the wiki namespaces from the forum bits and just rely on the Wiki tab, or you may want to show only one forum that has a Forum Link to the Wiki tab.