Make sure that your "select values" always match the array indexes and just do this:

Code:
  function getDetail() 
        { 
			var selctedItem = document.formDelivery.delivery.selectedIndex;
	
        // Initialise the text area prior to processing onChange Event for the control 
        document.formDelivery.textarea1.value = ""; 
        document.formDelivery.textarea1.value = DetailArray[selctedItem][1]; 

        }