Time to clear up the many wrong statements that people are making in this thread. I am the person who made the first powered PCIe extender and who documented why this is necessary:
http://blog.zorinaq.com/?e=44Firstly, the current drawn through the PCIe slot vary
a lot depending on the card. Dual-GPU cards usually draw more than single-GPU ones (contrary to what someone said).
That was me.
The amps you gave are probably for reference cards. I owned 2x Sapphire 4GB with the Accelero ventirad which have a custom PCB design and I'm pretty sure that they drew less power than a 5870: I could use a 6 GPU setup without problem (2x 5970 + 2x 5870) on a motherboard model where cards started disappearing from the system at 5x 5870 and eventually fried one board.
So my conclusion remains: unless you use always the same motherboard and card model, there's no way to predict what will happen.