For the 8800GT it is reccommended to have a 550 watt PSU for SLI but will have no problems running on a 500 watt PSU. But if you have some cheap PSU then it might have trouble but it shouldn't.
Check out the link in my sig for more info on power requirements for graphic's cards.
Note: The 9800 GT is just a rebranded 8800 so the requirement are about the same.