if i were you the better price vs performance, i'll go with the 2x 6950 unlock it to 6970 which beats 570 sli in almost all games(sometimes beats the 580sli depends on the resolution) for lower price/less heat/less watts and save about $420 , the unlock rate is very very high.
There are a several issues to consider on the 570 vs. 580 SLI issue:
2) CPU bottlenecking - is always higher on higher performing cards, and reduces the scaling factor of dual card setups to yield diminishing returns with higher performing cards.
3) Heat and noise - Fortunately, Nvidia seems to have figured this one out with new vapor chamber cooling and redesigned shrouds that allow better intake airlflow and airflow between cards.
GPU Temps (vs. single card)
6950: 89c (+12c)
560: 74c (+6c)
570: 81c (+4c)
580: 90c (+3c)
Asus 580: 73c (+7c)
In all, i would go with the GTX 570's in SLI, unless you have a REALLY high-end system and need absolutely the best. A 3D Vision Surround setup, for example, would definitely lean towards getting the best you can afford. Besides, being cheaper, the 570's will also yield better scaling to close the performance gap somewhat with the 580's.
Those reviews don't crank up the details full tilt for the most part. 580s will accommodate the 32xCSAA, 8x AA transparency (thanks for this tip matto17secs!) and the 16xAF with high tesselation settings. When you start getting into the details and then throw in higher resolutions, the 580s are the way to go in SLI.
I run them with an i7-930 with no OC. The regularly reach up 99% GPU load (measured through GPU-z) at times during BFBC2 sessions. They would not reach this level of utilization if there were a bottleneck.
If you want the best-newest-fastest card (cost not a consideration), there is no question.