If you could list the specific Brand and Model then we'd be able to inform on whether that it's sufficient. For an 8800GTX SLi setup an 850W gives you enough power to have a comfortable safe zone between the max power draw of your system and the max power your psu can deliver.
While something under 850W by a quality brand name could handle that system if you have the money, which by the setup of your system you do, then get the 850W. While some may say you need a 1000W pus for your system that is hardly the truth. Compared to your other components you want the psu to be of equal performance/quality, the psu is something you don't want to skimp on.