2x 5970s ~ 4x 5870s (UCed) ~ 4x 4870x2s ~ 4xGTX295s ~ 8x GTX260s (OCed) ~ 8x 4870s
So two 5970s would be the equivalent of EIGHT
4870s in Octo-Crossfire.
You are absolutely insane.
P.S. I'm just chugging along with a single 4870, so feel free to donate any GPUs or spare $$$ in my direction...