Dynamic form - how to

Hi,

I need a help from Drupal guru with deep understanding of Drupal forms and forms processing.
I'd like to build a form that can change itself by adding or removing fields, as a reaction on the appropriate buttons and then catch all the fields data in _submit function.
Let me illustrate the task with the code below. It is a managing system of the spy store, where operator can add and remove orders of the clients and store results in the DB.

// get the data array (see function code below)
$data = get_data();

// build the form dynamically according to the $data array
$form['client'] = array('#type' => 'textfield','#title' => t('Client'),'#default_value'=> $data['client'],'#tree' => true,);
$form['orders'] = array('#type' => 'fieldset',	'#title' => t('Orders'), '#tree' => true,);
foreach ($data['orders'] as $order_id => $order_description) {
	$form['orders'][$order_id] = array('#type' => 'textfield','#title' => t('Order '.$order_id),'#default_value'=>$order_description,);
}

// first two options should modify the form
$form['add_order'] = array('#type' => 'button', '#name' => 'op', '#value' => t('Add order'));
$form['delete_order'] = array('#type' => 'button', '#name' => 'op', '#value' => t('Delete order'));
$form['submit']   = array('#type' => 'submit', '#name' => 'op', '#value' => t('Save'));

$output = drupal_get_form('client_form', $form);
return  $output;


....

function get_data(){
	// get data from DB & return:
	return array('client'=>'Mr. Bond', 'orders'=>array(0=>'watch',1=>'knife'));
}

function client_form_submit($form_id, $form_values){
       // store updated order list to the database;
}

How this functionality may be implemented?
Thank you in advance,
Jacob.