For dual card setups, the key that should push your decision in favor of a pair of GTX 570's is their more effective cooler and improved case design. When placed in SLI, the 570 barely registers an increase in noise or heat. Comparatively, the 6970 (and modded 6950) get much noisier and much hotter when placed in Crossfire.
These are the temp and noise findings for a GTX570 SLI setup and a 6950 Crossfire setup (note that the 6970 Crossfire setup would be even hotter and louder):
GTX570 SLI (vs. single card)
Noise: 42Db (+2Db)
Temp: 81c (+4c)
"In SLI mode we do not get bad DBa levels returned once the GPUs really start to stress, roughly 42 maybe 43 DBa if the cards go really nuts. This means in the background you can hear fan and airflow, but very acceptable."
6950 Crossfire (vs. single card)
Noise: 49Db (+8Db)
Temp: 89c (+12c)
"In CFX mode we got pretty bad DBa levels returned once the GPUs really start to stress, on average the noise level was 44 DBa, however when the two GPUs really have a hard time then things get worse fast. In the Crysis WarHead benchmarks for example the cards spun up fast and reached 49 DBa of sound pressure, that's just downright noisy."
http://www.guru3d.com/article/geforce-gtx-570-sli-revie...
http://www.guru3d.com/article/radeon-hd-6950-crossfirex...
Also as Hardware Heaven describes (with noise samples): "As we have seen in the past the sales of a product can be hampered by a cooler which produces high noise levels. For the 6900 Series this isn't an issue in single card mode as the videos below show. The same cannot be said of the CrossFire testing though. As well as seeing temperatures up to 100c when we placed three cards side by side on our Gigabyte motherboard the noise levels being produced in dual and triple card mode were, to be honest, unbearable. They were similar to the GTX 480 at its worst in terms of volume but where the original Fermi card had quite a stable volume once maximum temperature had been reached the 6900 series is all over the place. Listen to the 6970 CrossFire video below and you will hear what we mean."
http://www.hardwareheaven.com/reviews/1084/pg21/xfx-rad...
Here's what Hardware Heaven had to say about two GTX580's in SLI:
"Earlier in the review we covered the fact that NVIDIA have gone out of their way to minimise the temperatures, power use and noise level of the GTX 580 following feedback on the GTX 470/480 range and the results above show how they have done in this respect. Overall the results are very good, with highlight being the fact that two GTX 580 in SLI are actually quieter than a single GTX 480. Power use has also been improved with the GTX 580 now operating in the same range as AMD's 5870."
http://www.hardwareheaven.com/reviews/1061/pg23/gigabyt...