A few things with BF3:
1.)On Ultra settings it seems 1.5GB is the minimum amount you need and the HD5970 2GB only has 1GB per GPU. You are hitting a VRAM limitation on top of any Crossfire scaling losses.
2.)BF3 testing in reviews is SP only as it is repeatable. Depending on the scene tested the difference between the HD7970 3GB and GTX680 2GB varies. The Going Hunting level(basically not much happens) shows the greatest performance gain for the GTX680 and other more intense scenes are smaller. However,the GTX680 is faster in all cases,but not always massively so.
3.)MP testing indicates the GTX680 is ahead but the problem in this case is that it is not repeatable,so how much better is hard to quantify with figures.
You can see the VRAM issue in the TPU review:
http://www.techpowerup.com/reviews/N...GTX_680/9.html
At 1920X1080 the GTX680 is not even 20% faster in the scene tested than an HD5970,yet at 2560X1600 performance collapses when AA is used. However,look how close the GTX680 and HD7970 are in the scene tested.
The TechReport uses another scene:
http://techreport.com/articles.x/22653/10
Anandtech is on review which uses Going Hunting and the difference is closer to around 25% or thereabouts.
There are a few things you need to also consider:
1.)The HD7970 3GB has had price cuts and cards can be had for around £350 to £360 if you shop around. OcUK were doing a VTX HD7970 3GB recently for around £333 delivered with a 10% off code. The card had a large 125MHZ overclock too(1050MHZ core instead of 925MHZ) and I suspect this would put it very close to a stock clocked GTX680 in BF3 but at the expense of higher load power consumption.
2.)AMD admitted even before the GTX680 that they clocked the HD7970 too low and one chap(Dave Baumann) said they would be looking at clockspeeds again. This hints that a faster clocked model is on the way.
3.)28NM production is expensive(Nvidia were complaining about this). However,prices will drop over the next few months as supply improves.
4.)This should mean the HD7970 3GB will probably drop closer to £300
IMHO over the next few months and no doubt so will the GTX680. I suspect more pre-overclocked models will be released of both cards
at standard prices as production moves to more non-reference models. You are already seeing this with the HD7970 3GB.
5.)The original GK100 was cancelled and replaced by the GK110. With the relative success of the GK104,this is most likely going to be released as the GTX780 or GTX685 at the end of the year to counter the new AMD cards.
6.)AMD tends to release new cards at the end of the year,and there should be a Tahiti XT sucessor around Christmas time or thereabouts.
If I had an HD5970 I would either "put up" with the HD5970 for the second generation cards,or pick up one of the current cards relatively cheaply at that time.