I think that your PSU is more than enough for that particular GPU and your system in general. But that is not to say that a 600W PSU might not help but I am obviously reluctant to suggest going out and buying a new PSU if at the end of the day you still have the problem.
Is the game just crashing to the desktop, or is your PC crashing? Do you get BSOD's if so what is the error message/code? Have you looked at the Events Manager to see if there are any clues there to the problem?