[Ailos]

I think 670 is a better fit for you largely because of the fact that NVidia's drivers are just better at the moment, as are the overclocking tools for Kepler.

The 850 W PSU is so overkill, you could put another 670 in SLI and still have your i5 overclocked (without the GPUs overclocked). Unless you plan on putting really aggressive overclocks on both GPU and CPU (taking the i5 to 5 GHz and the 670 to 1.5 GHz core), a 600-650 W PSU will be sufficient and much more cost effective. Taking either component to such level is going to need more than airflow though, so I don't really see the point.

The mobos are both way too expensive for these features. You can get any number of boards with all the same bells and whistles for $40-$50 less, even from ASUS and MSI themselves. Other good manufacturers (that I have personally had good experiences with): ASRock and Biostar. It just seems disproportionate to spend that much on mobo given how close that's getting to LGA 2011 territory, but if you go that route, a single card (either 670 or 7970) won't really do your system justice. So it's money thrown out into the wind, IMO.

EDIT: Just saw your new post above.
PhysiX is a physics engine supported in some games (including PS2). Its basic level allows realistic representation of in-game events, but an NVidia GPU can also run in a special mode to help out with the physics simulation and make things a lot more lifelike: things like water and fluids, fog, clothing and feathers, etc. In some games, having an NVidia GPU and enabling PhysiX can turn the eye candy level up to "ultra realistic".

As far as SLI/Crossfire: unless you know that you'll be doing this in the next 12 months, buying the 850 W PSU is not very cost-effective. You also need to keep in mind that both the 670 and 7970 can handle pretty much any game you throw at it with ease (even Crysis 3). To see the benefit of a multi-card setup with those two, you need resolutions higher than 1080p - things like triple-monitor surround and such. Such a setup will also almost always be processor-bound by the 3570K, and upgrading that will require the purchase of another motherboard later on, too, on top of needing an i7-type processor. Are you planning on those costs later on, too? If not, simply upgrading to a single GPU two generations from now (a GTX 970 or Radeon HD 9970 or whatever they'll call them) would be much more worthwhile.