First off what PSU do u have? As for AMD's new naming scheme it go's 6850, 5850, 6870 and then the 5870. As for which is better 5850 is faster but AMD has improved somewhat with the 6800's. Personally I would recommended going with the 6870 since it's about the same price as the 5850, and only a little more then the 6850.
@Red How do you find the noise levels on those ASUS/MSI cards? Currently I am very pleased with the performance of my XFX 6850, however the XFX HSF is a little louder then I'd like.