What motherboard are you using?
Whatever POS that came with the an Dell SC440.
Were you crossfiring the cards? Or in Windows XP 32bit (I think). Crossfire is forced to 4 GPUs max. One version of windows is reported to also have that limitation.
No crossfire, running under Ubuntu.
As for PSU problem, 6 5870s can run comfortably on a 1200W with no worries. The biggest worry is still as I said frying the motherboard by running so much power through it.
I'd also advise thinking about how you're going to get power to the 6pins.
ChrisB.