A 6850,as a single card,can do Eyefinity(3 monitors)but it doesn't have enough power to maintain good graphics at such a high resoultion by itself.You'll need to crossfire 6850's to be able to get playable framerates and have it look good.Otherwise your gonna have to play on the lowest settings.
SC2 is not even gonna be playable even on lowest settings.I also find playing anything that isn't a first person view to be somewhat dizzing.
What are you full system specs?(PSU,Mobo,CPU,etc.)