SLI:
SLI and Crossfire still suffer from microstutter and other profile issues. This topic has been beat to death elsewhere and almost all experts will recommend you get the best SINGLE card (single-GPU) you can afford. I can provide links if need be. There's a lot of confusion on this topic with many people thinking it's a very minor thing. It isn't.
4GB:
In case you don't realize, when you have two, 2GB cards in SLI you don't have 4GB as your frame buffer. Video RAM is cloned in SLI/Crossfire. If you had three cards, such as 3x (GTX670 2GB) you don't have 6GB, you have 2GB.
4GB needed?
Not presently for single monitors, even high-res ones. 4GB cards tend to cost more, so it's best to get a better 2GB card instead unless multi-monitor is on the horizon (then we're back to SLI/Crossfire).
SLI vs Crossfire:
SLI is superior to Crossfire overall.
AMD vs NVidia:
Pros and Cons, however the AMD games package presently offered is quite amazing. If you aren't committed to NVidia, a single HD7970 might be your best bet.
Four HD7970's to consider:
1)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125439
2)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127732
3)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125413
4)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202008
*GTX700 series:
NVidia has said these newer cards should have less of the micro-stutter/frame time issues. They should include an improved version of the frame rate metering technology to improve the SLI issues. Look for reviews specifically looking at these issues.
If you're dead set on SLI, I strongly recommend waiting a few months to see what happens. I suspect we'll also see more Video RAM as standard (such as a 760 3GB card).
Cheers.