By the original benchies, it seems the new cards will perform (synthetically) on par with the GTX480 and the HD5870. However, we are talking about the 5770 rebrand, the 6870, so, this should come out really interesting, specially, if AMD managed to get lower TDPs and temps.
Here are some TDPs listed but they aren't very specific nor reliable. Well, official info should be out next week since they will be released then