you need minimum 800W psu gold rated from my suggest
min psu wattage
6-pin & 8-pin
No, you don't need 800 watts to power a single card. 500 watts should work, but the power supply he has doesn't supply enough amps. 42 on the 12v+ rail is required, he has 38. While it might work, it isn't recommended, and it might just fry the card. Upgrade the power supply before attempting.
Hey. Thanks everyone for all the quick replies.
I just found this at Nvidia's site: http://www.geforce.com/hardware/desktop-gpus/geforce-gt...
According to that, you guys are right. I need a 600W min PSU. But... the card uses 250W at full load. Using Ohm's Law that would mean the +12V rails would need to supply 21 amps.
So, my PSU has the +12V current capacity, but doesn't meet the 600W recommendation. I may have to re-think this. The card is going in a Bitfenix Prdigy case with a 160mm-ish space front to back for the PSU.
I have a 750W and a 850W PSU sitting unused. But they are too long.
The recommended amps (42) is to account for your entire system. Your system might not push 650W under full load, but it will likely be more than 500. But you are correct, the most a 780 Ti would draw is around 250W.