The 6850 overmatches the 768 MB version but is pretty evenly matched with the 1 GB version. In SLI, I'd go with nVidia.http://www.guru3d.com/article/radeon-hd-6850-6870-crossfirex-review/15
So over the years Multi-GPU support has improved quite a bit, AMD still isn't up-to snuff at the level of NVIDIA though, multi-GPU supports still literally and directly remains the Achilles heel of ATI's Catalyst drivers. ......
But I would choose neither. With just 9 days till the release of the GTX 560 Ti on January 25th, it's an entirely new question that needs to be asked