1) Yes and no, the 6990 for example is very fast, but a pair of overclocked 2GB 6970s would be faster. Although I can guess that heat output and power draw may also be higher with 2 cards, worth considering as you will probably need a 1000W PSU witht he level of hardware you are looking at, and enough cooling to stop it all melting :)
2) This is a bone of contention. I would say that nV generally has better out of the box support, at least, their stuff doesn't need a display port screen, or adaptor. I cannot comment on drivers, but I had to format W7 to get CCC to install, so don't get me started on AMD drivers :P I have had great sucess with them in the past though. nV generally has faster support for new games with SLI profiles (or has in recent times). Might help you a bit.
3) This might be of some use.
http://www.hardwareheaven.com/reviews/1362/pg4/amd-radeon-hd-7970-crossfire-performance-review-battlefield-3.html
If you want to use max settings at 1080p x3 at 60fps, you need a lot of power, you are looking at a pair of 6970's or 7970's or 580 SLI, to get anywhere near 60fps. That also includes a GTX590 and 6990 card in that range. You really need the 7970 pair to pull of 60 fps, according to that benchmark anyway. If you don't crave 60fps constantly, or can manage on some medium/ high details, not ultra details, you could probably manage with 6950's or GTX570s.
4) I generally ignore those figures, and just look for bench performance. Fillrate doesn't generally mean anything, if it is not backed up with other equally as advanced bits on the card. High memory bandwidth without enough memory to pull of anti-aliasing for example, is useless.
My money would be on a single 7970, on a single screen, and I don't think you would be far wrong with a pair of them. But that would be around $1100 I guess, quite steep. My 2nd pick would be a pair of 2GB 6970's, IMO the extra cost (for me, would be £1100, compared to around £600) isn't worth it. I would drop the details a hair, and keep the higher frame rate personally.
All my 2 cents though! But whatever you do, DO NOT SKIMP ON COOLING OR THE PSU!!!!!!
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