Customize add form of a content type in drupal 6

Hi im using drupal 6.1
I would like to know how to cutomize the add form of a content type in drupal 6.
In Drupal 5 i used 2 write code in template.php as

if ((arg(0) == 'node') && (arg(1) == 'add') && (arg(2) == 'ebook'))
{
function phptemplate_node_form($form)
{
return _phptemplate_callback('ebook_add', array('user' => $user, 'form' => $form));
}
}

where ebook is my content type's name.

i did the same in drupal6's template.php but no change is happening.
Can anybody help me with this??