Custom layout for selected categories and posts?

I would very much like to be able to select some categories and have the layout of the category and the posts of those categories to be displayed using different template files.

It would be cool if I could make a template with the category name and the script automatically loads the template based on the category or posts category.

cat.site_news.php would load when the category "site news" is being browsed and if you wanted to customize the posts layout for the category maybe have a file called post._site_news.php