thanks Jason, getting closer.
i pasted your code, got a typo error and changed it (hopefully correctly) so it worked..
$cartNameCart_itemQuantity = floatval("".((isset($_POST["quantity_".$row_RsName['ID'].""]))?$_POST["quantity".$row_RsName['ID'].""]:"") ."");
i checked the page in firebug, and the id is correctly appended to the quantity field in each case, but the cart only updates the entered value from the first quantity field