If it's a good PSU, then 650W could handle 2x6970s.
Scaling is very very good now with driver improvements. You can expect to see anything up to 99% scaling, and in almost all modern games you're talking 60-90%.
With 3, it depends what multiplier the pci lanes are running at. With my x8x8x4 it is a bit watered down...but if you have a board that can handle full speed lanes, only your CPU will limit you (and at 1080 it will limit you, regardless of how powerful it is...3 graphics cards is a lot to handle
See this link
for information regarding scaling, and note you see the biggest increases at massive resolutions due to cpu limitations. Also note that AMD drivers have improved significantly since that review.