http://www.anandtech.com/show/4239/nvidias-geforce-gtx-590-duking-it-out-for-the-single-card-king
Anand's conclusion:
'If my final thoughts start sounding like a broken record, it’s because once again a set of NVIDIA & AMD product launches have resulted in a pair of similarly performing products.
The crux of the matter is that NVIDIA and AMD have significantly different architectures, and once again this has resulted in cards that are quite equal on average but are all over the place in individual games and applications. If we just look at the mean performance lead/loss for all games at 2560, the GTX 590 is within 1% of the 6990, however within those games there’s a great deal of variance. The GTX 590 does extremely well in Civilization V as we’d expect, along with DIRT 2, Mass Effect 2, and HAWX. Meanwhile in Crysis, BattleForge, and especially STALKER the GTX 590 easily comes up short. Thus choosing the most appropriate card is heavily reliant what games are going to be played on it, and as a result there is no one card that can be crowned king.
Of the games NVIDIA does well in, only Civ V is a game we’d classify as highly demanding; the rest are games where the GTX 590 is winning, but it’s also getting 100+ frames per second. Meanwhile on the games AMD does well at the average framerate is much less, and all of the games are what we’d consider demanding. Past performance does not perfectly predict future performance, but there’s a good chance the 6990 is going to have a similar lead on future, similarly intensive games (at least as long as extreme tessellation isn’t a factor). So if you had to choose a card based on planning for future use as opposed to current games, the 6990 is probably the better choice from a performance perspective. Otherwise if you’re choosing based off of games you’d play today, you need to look at the individual games.
With that said, the wildcard right now is noise. Dual-GPU cards are loud, but the GTX 590 ends up being the quieter of the two by quite a bit; the poor showing of the 6990 ends up making the GTX 590 look a lot more reasonable than it necessarily is. The situation is a lot like the launch of the GTX 480, where we saw the GTX 480 take the performance crown, but at the cost of noise. The 6990’s performance advantage in shader-intensive games goes hand-in-hand with a much louder fan; whether this is a suitable tradeoff is going to be up to you to decide.
Ultimately we’re still looking at niche products here, so we shouldn’t lose sight of that fact. A pair of single-GPU cards in SLI/CF is still going to be faster and a bit quieter if not a bit more power hungry, all for the same price or less. The GTX 590 corrects the 6990’s biggest disadvantage versus a pair of single-GPU cards, but it ends up being no faster on average than a pair of $280 6950s, and slower than a pair of $350 GTX 570s. At the end of the day the only thing really threatened here is the GTX 580 SLI; while it’s bar none the fastest dual-GPU setup there is, at $1000 for a pair of the cards a quad-GPU setup is only another $400. For everything else, as was the case with the Radeon HD 6990, it’s a matter of deciding whether you want two video cards on 1 PCB or 2 PCBs.
Quickly, let's also
Finally, there’s still the multi-monitor situation to look at. We’ve only touched on a single monitor at 2560; with Eyefinity and NVIDIA/3D Vision Surround things can certainly change, particularly with the 6990’s extra 512MB of RAM per GPU to better handle higher resolutions. But that is a story for another day, so for that you will have to stay tuned…'
