http://us.hardware.info/reviews/5623/12/nvidia-geforce-...
SLI Scaling:
1 card => 2 cards = + 88.4 %
1 card => 3 cards = + 124.6 %
1 card => 4 cards = + 156.9 %
2 card => 3 cards = + 19.2 %
3 card => 4 cards = + 14.3 %
Nothing really impressive, as usual with 3 and 4 cards (should get better with time / drivers) .... but 88% scaling with 2 cards, I'm impressed.