It wont happen its impossible (almost) to upgrade an alienware x51's psu as its specially made for all the alienware itself what im trying to say is even getting a new case etc is still almost impossible to do you would have to be very very very educated on computers to do so.
i have an alienware x51 myself and only the 240w PSU and it runs the geforce gtx 645 but not well i had problems when i first got it but MSI afterburner has helped its overheating issues but still its a hunk of junk either sell the one you have for cheaper then you bought or if you bought it within 30 days REFUND IT ASAP this computer is nothing but trouble and garbage sad to say :/