Customising the full page layout and sections based on node type

im using this snippet but its not working for me maybe im doing something wrong.

do i replace my entire page.tpl.php code with this one or do i add it to the top of the already existing code?

and secondly, how does this know if its my front page because my front page is just www.examplesite.com/ and not www.examplesite.com/front-page

/**
* This snippet loads up different page-type.tpl.php layout
* files automatically. For use in a page.tpl.php file.
*
* This works with Drupal 4.5, Drupal 4.6 and Drupal 4.7
*/
if ($is_front) {/* check if it's the front page */
    include 'page-front.tpl.php'; /*load a custom front-page.tpl.php */
    return; }
if ($node->type == 'book') {/* check if it's a book page */
    include 'page-book.tpl.php'; /*load a page-book.tpl.php */
    return; }
if ($node->type == 'blog') {/* check if it's a blog node */
    include 'page-blog.tpl.php'; /*load page-blog.tpl.php */
    return; }
if ($node->type == 'image') {/* check if it's an image node */
    include 'page-image.tpl.php'; /*load page-image.tpl.php */
    return; }
if ($node->type == 'forum') {/* check if it's a forum node */
    include 'page-forum.tpl.php'; /*load page-forum.tpl.php */
    return; }
include 'page-default.tpl.php'; /*if none of the above applies, load the page-default.tpl.php */
    return;