Unless the thing I'm missing is cooling. I haven't tried to build a 4 GPU system before. Is it going to be impossible to cool a 4x 5870 system that's running 24/7?
You would have to use flexible PCIe-extenders to connect four GPUs because if you plug them all directly into MB, they'll block ait intakes of each other.
And you can't plug them all directly because standart ATX system would accept only 7 cards (and each 5870 takes double space).