Geez, that's pure flash. I can't imagine why anyone would need four 6600GT or 7800-class GPUs. If you need 8 displays just use 4 cheap PCI cards (or 3 PCI and one PCI-E) with dual display outputs.
...except that they don't right now, because the proper drivers don't exist.
This setup points the way ahead but is currently useless. I'm not sure why we would want to go with these multi-GPU solutions since GPUs are still far away from the clock speed ceilings that currently plague CPU manufacturers.
Still, I can see NVIDIA releasing a new version of SLI allowing more than two cards to be daisy-chained...
But my god the cost. Four 7800s + a $250 motherboard and you still need a display, case, memory, storage....