I get the feeling 42A might be overstating the requirements by A LOT. It's probably to stop people with cheapo £20 "800W" PSUs that would blow up if connected to this card.
If the rating was accurate, I'd hate to see how much power the GTX 495 would draw!!!
Thats exactly what they are doing, as they would be in a world of bother if people started blowing up PSUs all over the place

I dug out the nvidia page that shows the PSUs that officialy support SLI in different configurations, again you can see they have built in a respectable safety margin... well except for the 300w psu thats listed for dual gtx295s, i think thats a typo that should say 1300w

http://www.slizone.com/object/slizone_build_psu.html