But don't 2 470GTX cards in SLI outperform a 580GTX?
Easily, but in the "real world" the 470gtx makes a huge loss for Nvidia, its been EOL type pricing for ages now, and is also for real EOL now so will run out in not too long. Great for the consumer, poor for business, and in real term sales, selling 10-20k cards in retail stores to end users for those kind of prices looks great but Nvidia/AMD don't want to go and sell something below cost in the 100k's to millions in volume to Dell and co.
We'll have to see where the 570gtx gets price, I imagine somewhere along the lines of earlier 470gtx pricing, IE, sucky, if the 6950 is around £250 and could beat it though, they probably can't maintain as high pricing as they'd like.
Right now(even more so at £165 a couple weeks ago) the 470gtx was exceptional value, a great buy. AMD's small core strategy and pricing mean Nvidia has little choice but to offer it at silly prices to clear stock.
Thing is, last gen, the gen before, the 4870 being the exception, in general the highest gpu is often just awful value, even compared to previous gen, the 470gtx/5850, the 8800gts 640, the x800xl, etc, etc, thats where you get performance vs value, the only real need for the highest end card is lack of ability to use two cards(with current mobo offerings, £5 extra on a mobo) means two slower cards will almost always outperform the highest end offerings for a lower cost.
The 580gtx is a boring card in that sense, theres a half dozen Nvidia and AMD options for better performance at a lower price, the 6970 won't be great value again being beaten in performance by things like 470gtx sli, 5850 xfire at current circa £150 a card pricing), etc, etc. The 6950 and 6990 though, could be outstanding value and performance.
Really the fight is between 6950, 570gtx and seeing how good value the 6990 ends up.