I read there is a paperclip trick to get a power supply to turn on without a motherboard.  I want to connect a power supply to an additional three 7950's connected to two different motherboards which already have power and video cards.  Ran out of connectors from the current two power supplies.  So, if I get a single power supply which will only be connected to three 7950's, how many watts should that power supply be?  

I was thinking about a 750 watt? or maybe 800?