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AMD today launched the Radeon HD 7990, which basically constitutes two Radeon HD 7970s on a single card. It's a little faster than a GeForce GTX 690, but also uses a lot more power, so if you're limited by power, the latter is the better buy. It's also more expensive than two Radeon HD 7970 GHz Edition cards while being slower, so if you're not sharply limited by power, two (or three!) 7970 GHz Edition cards make more sense. If you want the fastest performance you can get in a single slot, the older unofficial 7990s from PowerColor and Asus are clocked higher, so the new 7990 is slower than those. If you want max performance at any price, then two or three GeForce GTX Titans is the way to go, as that has considerably faster GPUs.
So basically, AMD just launched a new card and I don't see any reason why anyone would want to get one. But hey, it's finally here after being delayed by a year or so from the initial rumors.