Do not try to run two 6990s on a 850 watt PSU, it will fail, even a 1000k watt power supply it will cause issues for sure, unless you have a very low power rig in general for the rest of the rig.  Its not just the two cards, but I have seen the ones I have built blow the thermaltake 1000watt power supplyes a couple of times already, its just a lot of stress on the PSU if you run it for mining 24/7 with the rest of the system and the heat.  It can be done for sure, but your much better off having some room to play with.  Definitely do not go lower then a 1000 watt PSU for mining with those cards.