Originally Posted by zenyora
future builds (i7 ,dual cards etc )
The GTX 580 needs, according to guru3d:
GeForce GTX 580
On your average system the card requires you to have a 650 Watt power supply unit.
GeForce GTX 580 in SLI
A second card requires you to add another 250 Watts. You need a 900+ Watt power supply unit if you use it in a high-end system (1 KiloWatt recommended if you plan on any overclocking).
They suggest a 750w for the 6990, so might be able to swing a 1000w for Xfire, though it may need to be a bit juicier.
So, 950-1000w for GTX 580/HD 6990 in SLi/Xfire.
Possibly a 1200w will be able to SLi/Xfire the next gen.
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i5 3570k | 8GB G.Skill 1600MHz | R9 290 | SeaSonic 750X