^ I think you mixing up the 5770 and 6770. The 6770 is basically the same as the previous 5770 and was equal to a 5870. Then ATI changed their naming scheme the top end are now 6950 and 6970.
It depends on the game, some do not scale well with multiple cards. It is about the same as 6970. Take look here
lol tcfraz edited his message while I was typing the first paragraph does not make sense now.