That naming convention will work, just not by default. Assuming this is Drupal 6, try adding the following code to your theme's template.php:
/**
* Override or insert variables into the node templates.
*
* @param $vars
* An array of variables to pass to the theme template.
* @param $hook
* The name of the template being rendered ("node" in this case.)
*/
function yourthemename_preprocess_node(&$vars, $hook) {
$node = $vars['node'];
$vars['template_file'] = 'node-'. $node->nid;
}
Make sure you don't try to redeclare yourthemename_preprocess_node()
--that is, if it already exists in your theme's template.php, just add the $node and $vars['template_file'] lines to it.