I'd buy on features more than price/performance. Not sure how eyefinity stacks up against nVidia surround. Eyefinity is more flexible, but for 3 monitors either should work fine.
One thing I'm not sure if you're aware of, the 6870 is not the successor to the 5870, they've messed with their numbering scheme. It's in between the 5850 and 5870 in performance (and somewhere on par with a GTX470.)
Yes, but what about GPU temp and noise? Currently my dual 260 GTX are just jumping to almost 80 Degrees when after 2 minutes of gameplay.... Will the single ATI card be less noisy and will it suffice to run the game in it's full potential?
WoW is quite CPU limited depending where you are in the world, Dalaran won't give earth shattering FPS without a good CPU (unless things have changed from ICC patch (when I quit)), so you might not gain too much in the way of FPS. 25 man raids are also very heavy on the CPU.
Currently running a i7, with a good Watercooling solution. I haven't had any issues in the past when raiding and everything was smooth.