Personally if it was me id go 6970's over the 570's
Cost efficient + less power consumption with good performance
I dont think there would be much diff between 6950CF vs 6970CF so maybe picking up some 6950's would be a good idea
Also if you did choose to go the 6950 route I'd recommend the MSI 6950 OC Twin FrozR III's, with some good case cooling those things never get too hot. My card never goes over 56° and sits at 34° idle