How much wattage?

Geforce recommends 600w PSU for a system containing a GTX 780: http://www.geforce.co.uk/hardware/desktop-gpus/geforce-gtx-780/specifications

Geforce power recommendations are typically "overkill" to cover themselves, but I always stick to their recommendations anyway. Better to have to have plenty of power in reserve than using a PSU which is running at it's max output all the time.
 
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