In drupal 6 you can theme the individual content types easily. Suppose you have a content type called 'company'. So go to your theme folder and create a php page 'node-company.tpl.php'. Now place your company related theme in this page.You are done.
Thus in short if you want to theme individual content types, create a php file named 'node-<your content type name>.tpl.php' and add your content type theme here.