>No ones answered the question that the z840 only comes with one 6pin power cable avail.
The Z800, Z820 and Z840 comes with two (2) 6-pin connectors. The second one is attached inside the chassis near the HDD´s in the frons side of the chassi and can be trixky to find if you don´t know where it is, but it is indeed there.
Yes, the card will work with the 800 watt PSU.