NVIDIA's new GeForce GTX 590 sets out to conquer the dual GPU performance throne. It battles AMD's Radeon HD 6990 that was released just two weeks ago. Designing this kind of graphics card means you are limited by power and heat instead of by the raw power of your GPU design. Basically you decide on the power input configuration, 2x 8-pin in this case, which means the card is specified to draw a maximum of 375 W (= 150+150+75) of power. Then you find a thermal solution that can move that kind of heat away from the card. In the next step you build the board and tune the clock speeds and voltage to fit into those limitations. A more power efficient GPU will let you clock it higher while staying within the power limit, which is what seems to be the deciding factor in the GTX 590 vs. HD 6990 duel. Based on our testing AMD's Cayman GPU is about 20% more efficient than NVIDIA's GF110, which means that AMD can run their GPU faster, letting it catch up on the single-GPU performance disadvantage it has. Now with two GPUs in place, single GPU performance doesn't matter anymore but performance per Watt.
As a result GTX 590 and HD 6990 are roughly the same performance when averaged. In lower resolutions GTX 590 wins, at 1920x1200 both are even, and at 2560x1600 AMD's HD 6990 wins by 3%. Since 2560x1600 is the primary resolution that both of those cards should be used for, my conclusion is that GTX 590 is slower than HD 6990, but not by much. It is still disappointing to see that NVIDIA could not turn their single-GPU winning GF110 into a dual-GPU design win too.
While I usually do not believe in large gains from new drivers, this time I am not so sure. If NVIDIA's power limiting system is active all the time, in all applications, then this could cause some applications to get capped down, hurting performance a lot. If NVIDIA manages to tune their algorithm some more, they could get some extra performance out of their design - without the typical driver/shader optimizations that we see from time to time in graphics drivers.

Both AMD's HD 6990 and NVIDIA's GTX 590 are hard to tame beasts during overclocking. On AMD you need to disable their ULPS ultra low power state to get anywhere. On NVIDIA I couldn't find a fix for the extremely unstable card during overclocking. When increasing clocks to find stable settings, the card often completely crashes the system in a test that it completed before, with Windows 7 the normal behaviour should be that the driver will stop rendering, show a "The driver has stopped responding" message, and you can just continue without reboot.
Once I got normal overclocking stable I went to work on voltage tuning which promises additional performance gains. Within a short time of testing, the card broke permanently by what seems to be an overcurrent situation. NVIDIA promises that their power capping will avoid such conditions by constantly monitoring the card's power draw and reducing clocks if necessary - in all applications. Apparently that system did not work and took the card with it. At $699 and possibly an RMA denial if that happens to you, my suggestion is that you should really not engage in overclocking with GeForce GTX 590 until this issue has been completely cleared up. For the record, the card worked perfectly fine and rock stable when it was just installed and left alone without any extra tweaking.
When directly comparing GTX 590 and HD 6990, it is difficult to find many clear wins for the GTX 590. It does emit a good deal less noise but is still noisy, all the other comparisons are either a draw or win for the HD 6990. Feature wise NVIDIA does bring a bit more to the table with 3D Vision Surround, CUDA and PhysX - if you use those. We wanted to bring you a Quad-SLI review of two of these cards in SLI, but NVIDIA did not have a driver ready. Quite surprising considering that this card is not much different to a GTX 480 Quad SLI setup. Functionally GF100 and GF110 are identical, the configuration of two GPUs on one card connected by NF200 are functionally the same as two cards connected by an NF200 chip located on the motherboard.

Price-wise both HD 6990 and GTX 590 are tied around $700 which is a lot of money to spend on a graphics card. My recommendation would be to go with a single GPU GTX 580 and wait what the future brings in terms of games - most games are console ports, Crysis 2 is DX9. Developers! The PC needs more love from you.