Is 850w enough?

Would you please specify exactly what your computer parts are?

or you can check out this site and fill in the details for yourself...

http://www.thermaltake.outervision.com/

as long as its a decent brand of psu that should be more than enough.
the gpu's use between 130 to 250 watts each your cpu which isnt listed will probably use between 95 and 140 max
5-10 wats for the fans 40 for the dvd roms and 60 for the other hdds and 20 max for the motherboard your still inside 850. and thats with everything maxed out.
so your average usage will be around 370-450watts

Yes a quality 850W psu will be more then sufficient.
http://www.guru3d.com/article/radeon-hd-6950-crossfirex...

What brand and model is the PSU, which means, is it 850W, or "850W?" If it is a quality brand that can actually produce 850W, you'll be fine. If it is a Chokemax with a liar-label on it (e.g. Diablotek), it will blow chunks and / or damage attached components.

850w is more than enough, usually psu's 800w or more are for servers. even a branded 600w would be enough to run your system, 850w is too big for that, unless its a branded 850w, its a true power 850w, but if its a generic one, it wont.

guys im talkin 2 2gb radeon hd 6950's
these are monster cards that will be in crossfire
and to top that off i'm unlocking their bioses to run as 2gb radeon hd 6970's
you cant tell me that 600w is enough to power those with evrything else in my system.

plus its an A-power psu, not realy much of a brand but it did hold out much longer from blowing chunks against 2 radeon hd 5770's in crossfire where as the GIGABYTE ODIN PRO 800W lasted 3-4 months (last year) before blowing it main capacator.

like i told you, "BRANDED" 600w can run your system not generic. its the minimum, but ofcourse you wont go for the minimum, you always wanted more than a minimum.
processor and graphics card are the only 2 that eats more power from your psu, everything else uses small amount.
top graphics card eats 150-220w, your processor 80-100w, and everything else like hardrive, memory, mainboard uses very small wattage.
about 250w is being used overall excluding a graphics card, if you have true 600w, you still have 350w left for the gpu, thats alot and can handle sli, crossfire.
i am using true 600w psu for 5 months, powering my i7950, sli.
if you have a generic 850w, its not enough, a branded 700w is even better.

heres your guide.
http://www.tomshardware.com/reviews/geforce-radeon-powe...

Although two years old, the guide does illustrate the point. Look at the current and recent SBM articles for more measurements of actual power use.
GPU makers overstate requirements in an attempt to account for the crap PSUs out there that can only output 50% of their labels (e.g. Diablotek & Logisys).