$orders = Mage::getModel('sales/order')->getCollection();
Mage::getSingleton('core/resource_iterator')->walk($orders->getSelect(), array(array($this,'proccessOrder')));
public function proccessOrder($args)
{
$order = Mage::getModel('sales/order');
$order->setData($args['row']);
echo $order->getIncrementId() . '<br/>';
echo $order->getGrandTotal() . '<br />';
$orderAmount = $orderAmount+$order->getGrandTotal();
echo '----------------------<br/>';
}