How to get the node id before saving the node in drupal 6?
You can use hook_nodeapi presave option to get node content before saving the node.

Example : 

function MODULENAME_nodeapi(&amp;$node, $op, $a3 = NULL, $a4 = NULL) {
  switch ($op) {
    case 'presave':
      print $node-&gt;nid;
      exit;
    break;
  }
}