All right, so I'm looking to build a semi-cheap (under $1000) gaming rig and I'm just trying to weigh my options here. To go with my i5 2500K with 8GB of ram, I'm looking at either a pricier, factory OC'd 560 Ti or two cheap-ish 6850s for crossfire. Considering the latter really wouldn't be that much more expensive if I went with two of these I'm leaning toward the latter.
The 6850's would perform better overall, but you do have to deal with some dual card issues; sometimes crossfire doesn't work and micro-stutter can be noticeable on two card solutions(I have never used dual cards personally; I just know about it and understand why it might happen).
The 560 Ti would be more consistent in performance.
Both setups would play today's games well. I use a 6950(equivalent to the 560Ti) and I have played BF3 with almost every setting turned up@1080p.