last i heard, crossfire didn't work well with eyefinity. this may have been fixed though, so you may want to check the latest CCC release notes.
since the 5770 crossfire performs similarly to a 5870, (assuming the eyefinity performance is not gimped) i woudl expect most games to be playable at that res.
BC2 is one of the more demanding games, but at medium-high settings you should get playable rates. luckily eyefinity doesn't hit framerates too badly, msot games will still max out at eyefinity resolutions on a 5870.