The 5770 is at least 2 times better than 3870 (correct me if I'm wrong). But do you need it? 3870 is still a good card. What resolution is your monitor?
And also, it's the first time I see a passive 5770. I researched it a bit. It was released in August, but there is nothing like this on newegg. Maybe it runs too hot? Is there going to be any air flow inside the case?