Example of how to add an autocomplete path to a textfield and convert it to the user ID before saving it to a variable via `system_settings_form()`.

D6MODULE_user_autocomplete.php

<?php

/**
 * @file
 * Example of how to add an autocomplete path to a textfield and convert it to
 * the user ID before saving it to a variable via `system_settings_form()`.
 */

/**
 * Admin settings form.
 */
function D6MODULE_admin_settings_form() {
  $form = array();

  $form['D6MODULE_user_autocomplete'] = array(
    '#type' => 'textfield',
    // Drupal core already provides an autocomplete path for users.
    '#autocomplete_path' => 'user/autocomplete',
    '#value_callback' => 'D6MODULE_admin_user_autocomplete_value_callback',
    '#element_validate' => array('D6MODULE_admin_user_autocomplete_validate'),
  );

  return system_settings_form($form);
}

/**
 * Value callback for the `D6MODULE_user_autocomplete` field.
 */
function D6MODULE_admin_user_autocomplete_value_callback($element, $edit = FALSE) {
  if (func_num_args() == 1) {
    // No value yet, so use #default_value if available.
    if (isset($element['#default_value']) && is_numeric($element['#default_value'])) {
      // Convert to autocomplete value.
      $account = user_load(array('uid' => $element['#default_value']));
      return isset($account->name) ? $account->name : '';
    }
    return '';
  }
  return $edit;
}

/**
 * Element validate callback for the `D6MODULE_user_autocomplete` field.
 */
function D6MODULE_admin_user_autocomplete_validate($element, &$form_state) {
  if (!empty($element['#value'])) {
    // Check that the value is a valid user.
    $uid = db_result(db_query("SELECT uid FROM {users} WHERE name = '%s'", $element['#value']));
    if (!isset($uid) || empty($uid)) {
      form_error($element, t('User must be a valid user.'));
    }
    else {
      // If no error, set the value on the element to the validated uid.
      form_set_value($element, $uid, $form_state);
    }
  }
}