I'll second that Sparkle Calibre suggestion. The EVGA you listed is also an excellent card. The GTX 460's that have been mentioned have really high factory overclocks, compared to the 6850's that are only modestly overclocked. Out of the box, the 460's will be faster, cool, and quiet with excellent dual-GPU scaling.
The 6850's will also do well in dual card setups, but some would say their drivers are lacking. In general, after browsing these forums, Nvidia drivers seem to be more stable in SLI, while AMD has several commonly identified issues with shimmering and microstuttering. AMD also requires you to install application profiles in order to enable Crossfire support in the latest games, while Nvidia does not. You are fortunate to have an SLI certified motherboard, so I would say take advantage of that.