And even maxing out the 12 volt power is unrealistic. According to my Antec 650 w TP3 box, it has three 19 amp 12 volt rails. (I'm not sure that's true, because I read continuity between any two 12 volt lines.) It also says total 12 volt power is 52 amps (624 watts). 26 watts aren't enough to run the rest of a computer.
The rumored 580 watts for the video card just doesn't pass the, "Does this make sense?" test. If I have my specs right, PCI 2.0 - 15 amps, 8 pin PCI-e connector - 15 amps, and PCI-e 6 pin connector - 7.5 amps for a total of 37.5 amps. That equals 450 watts. And if the card is usable in a PCI 1.0 slot, that drops the worst case power draw to 360 watts.
In that case, using an 8 pin PCI-e plug makes sense. Two 6 pin PCI-e plugs and a PCI 2.0 slot would have the same capacity as one 6 pin, one 8 pin, and a PCI 1.0 slot.
With all that in mind, 580 watts does seem like a reasonable estimate for total SYSTEM power consumption.
But then, I can prove Heaven is hotter than Hell also.