Wiki landing page

When i goto to my wiki at http://www.exousialinux.org/wiki i get the listing of posts as standard. But how can i have a specific php page run instead say i made a page called wiki_main_page.php how can i have that load up instead of the default posts lists ??

I am quite happy to get my hands dirty in php and css as i had to customise the css etc to get it to fit in my theme as you can see from the test page at http://www.exousialinux.org/wiki/test

Regards,

Justine