Well my rigs with two 6950s, plus system components (single core AMD sempron and 1 gb ddr 2 ram), use almost exactly 500w from the outlet (confirmed by kill-a-watt meter). I'm using 80 plus certified power supplies that are around 80% efficient, so that means that in actuality the cards are using 400 W of the power supply's rating (I believe they're rated on draw from parts, not draw from outlet but I could be wrong). Anyway, the chip is only using about 30 W of that and the rest of the system maybe 20 (just guessing), so that means each card is probably taking about 225 W from the power supply.

Now a 6990 is two chips, and has roughly the same draw as two of my 6950s. So that means it probably uses around 400 W each card. Given that you want a little breathing room, I would say you can't put more than 3 cards on one 1500 W power supply.

The better idea I think would be to buy two cheaper 1000W power supplies to run four cards (if you want to go the 4 card route).