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Two Radeon HD 7970s at 1 GHz on a single card should easily trounce a GeForce GTX 690 in raw performance, since the GTX 690 had to underclock the GPUs considerably from GTX 680 reference speeds in order to fit inside a 300 W power envelope.
The trouble is that the way it gets the extra speed is by using enormous amounts of power. The card comes with three 8-pin PCI-E power connectors, which means that it would be allowed to draw as much as 525 W. It also uses a huge 3-slot cooler with three fans to cool the card. PowerColor says the card can handle 550 W, which I find plausible but not obvious.
And then the other problem is the limited utility of dual GPU cards. If you want two GPUs in SLI or CrossFire, you're better off getting two single GPU cards. That's probably also true if you want three GPUs. So the Radeon HD 7990 is really meant for people who want four GPUs in quad CrossFireX.
And so we go back to power consumption. Two GeForce GTX 690s mean 600 W in heat from the video cards. Two PowerColor Radeon HD 7990 Devil 13s mean what, maybe 1000 W? Add in a processor and power supply inefficiencies and you're looking at a 1200 W space heater of a computer. Is that what you really want? I don't.
So it looks like this generation will basically end up as the reverse of the previous one. Last time, the Asus Mars II (two GTX 580s on a single card) was the fastest dual GPU card on the market, but the Radeon HD 6990 was far more sensible because it used so much less power. This time, some variant of a Radeon HD 7990 (possibly but not necessarily PowerColor's) will be the fastest card, but the GTX 690 is the more sensible one, again because of an enormous disparity in power consumption.
Unless, of course, you want to use more than four monitors, in which case, I'm not sure if GTX 690s can do it. A single GK104 chip can only do four. I'm not sure if you can use Nvidia Surround with more than four monitors by plugging them into different cards. The GTX 690s each have four monitor ports, and you'd think that if the cap were four per GPU, they'd have some monitor ports wired to each GPU to allow the usage of more monitors.
Does it sound crazy to talk about wanting to use more than four monitors? Well, consider that we're talking about using four GPUs, and doing that on a single monitor is simply nuts, with the possible exception of a quad HD 3840 x 2160 monitor. Except that AMD supports that in Radeon HD 7000 series cards and I don't think that Nvidia does.
The upshot is that dual GPU cards are a very narrow niche market, and while some variant of a Radeon HD 7990 will be the way to get a quad CrossFireX setup with the highest graphical performance possible, that only makes sense for a tiny handful of people in the world. And in particular, it doesn't make sense for most of the people who will buy a 7990.
If I had to have a dual GPU card, I'd rather have a GeForce GTX 690 than a Radeon HD 7990, due to the power consumption difference. But I don't want a dual GPU card at all. If I had to buy a new card right now, I'd probably grab a Radeon HD 7870 or so.