How can I use node_load with a module created page that requires an argument? I would like to load the contents of a page like how node_load() works, although what I am trying to load isn't a node but a page created using a module. The entry in hook_menu uses an argument which needs to be passed in the URL. This is for a popup (jquery overlay) that I want to reuse where the code is already done for the standalone version.

Any idea how this can be done? (Drupal 6)
The code executed from Drupal, in index.php, contains the following code.

$return = menu_execute_active_handler();

// Menu status constants are integers; page content is a string.
if (is_int($return)) {
  switch ($return) {
    case MENU_NOT_FOUND:
      drupal_not_found();
      break;
    case MENU_ACCESS_DENIED:
      drupal_access_denied();
      break;
    case MENU_SITE_OFFLINE:
      drupal_site_offline();
      break;
  }
}
elseif (isset($return)) {
  // Print any value (including an empty string) except NULL or undefined:
  print theme('page', $return);
}


menu_execute_active_handler() accepts an argument: the path being shown. If that argument is not passed to the function, then menu_get_item(), the function called by menu_execute_active_handler() uses the content of $_GET['q'].
In your case, you would pass to the path you are interested to. Your function should use code similar to the following one:

function mymodule_get_page($path = NULL) {
  $return = menu_execute_active_handler($path);

  // Menu status constants are integers; page content is a string.
  if (is_int($return)) {
    return $return;
  }

  if (isset($return)) {
    return theme('page', $return);
  }

  return −1;
}


The calling function would need to check the returned value: If it is a string, then it the what returned from theme('page'); if it −1, then the menu callback didn't return any value (but could have printed the page content; if it is an integer different than −1, then it's a Drupal error code, i.e. the one reporting the site is off-line, the user is trying to access a page without to have any permission, or the user is trying to access a non existing page.

The only "problem" with this code is that sometimes a menu callback doesn't return any content, but it prints the content directly; this is normally done for content that doesn't need to appear together the rest of the page output Drupal normally adds (e.g. the content for the blocks associated to a page), such as when a module returns XML content that should not mixed up with the HTML output generated from Drupal.
In this case, the function should intercept the output directly printed, with ob_start() and ob_get_flush().

function mymodule_get_page($path = NULL) {
  ob_start();
  $return = menu_execute_active_handler($path);
  $output = ob_get_flush();

  // Menu status constants are integers; page content is a string.
  if (is_int($return)) {
    return $return;
  }

  if (isset($return)) {
    return theme('page', $return);
  }

  return $output;
}