to be honest, 6950 cfx > 570 sli and its cheaper
Read the official tomshardware charts in the earlier post 570sli > 6950. The next step up in ATI is 6970cfx which is same price point as the 570sli and as you can see the performance is better with the 6970s. The only thing is with AMD video cards there is less driver support. No physicX. And when in crossfire the drivers don't support 3d gaming, the sli does.
I actually purchased the 6970 CF and they were awesome in raw power but when it came to driver support they suffered, had to exchange them.