Depends on the game, I've seen it 30% faster in one title and 40% faster in a benchmark, overall it looks to be anything from 15%-30% faster on average.
Turns out I was right, 15% on average when 30% ahead of GTX580 in Crysis. Simple maths does miracles

Lets hope it will drop to £349 soon after launch and I'll get my hands on one of these.
BTW, if it beats HD5970 then it's a very much step up by AMD. Not a refresh at all.