For SLI, I'd probably go with the ASUS DC2 - it's one of the coolest and quietest cards. You could always overclock it a bit if you wanted it to match the FTW's speeds.
The EVGA is certainly a very good card. When I buy Nvidia again, I'll probably go EVGA just because I've always had a great experience with them. Also, if your case doesn't have great airflow, you'll want the EVGA as it exhausts the heat out the back, while the ASUS will exhaust all its heat into your case (not a problem if you have good airflow but a huge problem if you have like 1 exhaust fan).
What do you mean by "3D rendering"? You can't really compare specs like that, but yes the 7970 is far superior at GPU Compute tasks than the 670 is. So if you're using the cards for something else in addition to gaming, the 7970s might be an easy choice. I don't do any GPU Compute tasks myself so can't comment too much about it.
I'm still trying to find a recent review that would directly compare 670 SLI to 7970 CF at 3 screen resolutions but am coming up blank. I know that 7970 Ghz ed will be faster but I'm not sure exactly by how much. I think 15% is a safe guess.
3D rendering such as Maya, 3Ds max and photoshop.
How is the game support/driver of the ATI compare to Nvidia?