The x850 in my opinion was just a way for ATi to pull slightly ahead of nVidia in performance. Clever marketing scheme, that's all. Same can be said with SLI though, people are of course instantly gonna think "two cards! Double peformance here I come!" I think ATi saw nVidia's weakness in their time devoted to SLI and decided to focus on one single, high performing card (r520) before competing with nVidia's SLI. I dunno though, that's just my guess.
SLI is here to stay unfortunetely, it's gotten very widespread now with many different motherboard manufacturers supporting it in a variety of enviroments, and with ATi taking up arms against it now I don't think either company will back down with it.
And this engine will be something else, that's for sure, although I still haven't seen much of it apart from basically what it is. But still, anyone worried with a 6600GT about not being able to play it has NOTHING to fear, I don't think it'll be that intense (although at the same time I'd like it to be because then you know the graphics are gonna be orgasmic :P).
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w