for the z800 (and z820) systems HP requires the optional 1100 watt supply when using two high power cpu's (over 120 watts)with a high end HP approved video card, and either a sec video card or video coprocessor, 4 HD's and all memory slots filled
the stock 850 watt supply is for one or two 80 watt or lower cpu's one hp approved video card cd rom, 2-3 hard drives and half, to all memory slots filled
what you have to understand is that because the HP power supplies are multirail, removing the total draw from the CPU 12 volt rail will not help with the wattage draw on the 12 volt GPU rail
in a multirail power supply each voltage is seperate from the others, and you can not draw more that the max stated amps
from each rail so even if the CPU 12 volt rail is under the max wattage/amps, you can still trip the supply off if the draw on the GPU 12 volt rail is to high
the HP z800/z820 power supplies pop out very easy and should have a label showing each rails max amps