Once upon a time one developer was very upset, because he coudn't find appropriate templates name for drupal commerce checkout pages. And he began his research. Here is the result of his research.
Checkout page template: page--checkout--%.tpl.php
Checkout review page template: commerce-checkout-review.tpl.php
Page checkout complete: page--checkout--complete.tpl.php
Also will be usefull to know about user order (! one order page!!!!!) page template: page--user--orders--%.tpl.php