I got a bit unsure now and looked through the quick specs and didn´t find anything that said that *how many* six pins Z840´s with the 850W PSU actually have.
It stated that 1125W PSU should have three six-pin connectors. (so i was wrong there)
However, Nvidia recommends a 600W PSU so your 850W should do the trick. If you have one six-pin connector you can use that for the card and then use the molex contacts in the computer to make a 8-pin connector. 