The category description is already displayed in the category view page.
You just need to move elements around.
The template you need to modify is this:
app/design/frontend/{package}/{theme}/template/catalog/category/view.phtml.
take this code:
Code:
<?php if($_description=$this->getCurrentCategory()->getDescription()): ?>
<div class="category-description std">
<?php echo $_helper->categoryAttribute($_category, $_description, 'description') ?>
</div>
<?php endif; ?>
and move it at the bottom of the file. That's it.