Here is how to connect to a database other then the Joomla database using the Joomla 1.5 API.
$options = array();
$options['driver'] = 'mysql'; // or mysqli
$options['host'] = 'localhost';
$options['user'] = 'j10';
$options['password'] = 'j10';
$options['database'] = 'j10';
$otherdb = JDatabase::getInstance($options);
$otherdb->setQuery('SELECT * FROM jos_users');
$rows = $otherdb->loadObjectList();
print_r($rows);
The $option values are dependent on your database settings.
