Interesting review
Anandtech came to a similar conclusion regarding price/performance of the 5770, but they seemed to quite like the 5750 because it ran so much cooler and performed much in line with the 4850 (conclusion: http://www.anandtech.com/video/showdoc.aspx?i=3658&p=14).
Guess it all depends on what you want out of it.
Me, I only just got a 4890 a few months back, so no plans to change yet!