i'm not sure how to do it either but i wanted to have this kind of page too.
i thik joomla has this feature.
Category Page (WP plugin) # Pixline
This plugin allows to connect Pages to Categories, and let you display a Page’s content on top of category archives. It’s a perfect companion for WordPress, especially if you are planning to use it as a CMS system, more than a blog.
It let you write a Page, and use its content as a fixed header for your category archive, using a special template tags. It will not help you to show some kind of category-related content in your pages or elsewhere,
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Will.Spencer (17 October, 2009)