Why do you need to know, 8800 gt uses 105 watts max under full load, great efficient card, your not even close to your limit. I have 450 watt psu ultra quiet. Look at my specs, I am not maxed, I use 290 watt under full load in game multitasking. PSU's are so misunderstood, the companys put these things out in response to a demand for power a while ago. So much current hardware runs more efficiently then a few years ago and it will just get better, 850 watts is overkill, your not even close. Here is a guess, at idle you use 175 watt and under full load between 275 and 390 watt, I bet you. make sure to post back and tell what you got for usage Tower only. Use a plug in reader, run your power cord from TOWER to it then to plug in.
You could SLI 8800 gt with 2 more hardrives and 2 more gb of ram and be fine, Overclocked and all. and because of the 1 pin power for 8800gt there would be no problem with 12 or 5 volt rails
OH YA just read about this it might be out and tell you what your usage is.
Nvidia's Enthusiast System Architecture (ESA)
software that gives you all kinds of system info.
GOOD LUCK
PS: Do you crash with your gt so overclocked? I don't but some people do when overclocked as much as yours. and its not a psu problem, most forms point to problem being with the ram speed oc on 8800 gt.