Are you from Europe McFly? Cause every Dell Rep I asked for an upgrade said, that the upgrade will probably only be available in the USA and not in Europe. Also Markus Schütz (Senior Press Reviews Manager Dell Europe, Middle East, Africa) stated in a German forum, that an upgrade in EMEA isn't planned.
Seems the only way to get the 8800M GTX Sli is to wait until they appear on ebay or to sell the M1730 and get a new one instead.