How powerful should my power supply be?

I'm planning to get the Phenom 955 be and overclock it. My RAM would be 1600 DDR3, GPU 5850 x2 CF. In the future, I'll probably upgrade to the next generation of AMD CPU.

What watt PSU should I aim at?

http://www.corsair.com/psufinder/default.aspx this one said corsairs 950 to 1000

http://educations.newegg.com/tool/psucalc/index.html this one said 836W PSU

http://extreme.outervision.com/psucalculatorlite.jsp this one you can do

but most will say a good 750 will do

i would say with that CPU and OC ing and xfire do a 850 Corsair