Article about that on the front page...
Single Card:
R9-290X
GTX970
GTX780Ti
GTX980
GTX690 (dual GPU)
R9-295X2 (dual GPU)
Titan Z (Dual GPU)
Multi-GPU
Pair of R9-290X
Pair of GTX970
Pair of GTX980
Tri R9-290X
Tri GTX970
Tri GTX980
Pair of GTX690 (4 GPUs)
Quad R9-290X / Pair of R9-295X2 (4 GPUs)
Pair of Titan Z (4 GPUs)
Quad GTX970
Quad GTX980
More or less an extrapolation. Not very good return on investment past two cards.