Here's a link to a PSU calculator
. I just guessed on a few things, but it looks like you should be fine.
A single 5770 puts you at about 280W. Cross-fire puts you at about 340W. It is a 80 Plus Cert. PSU, so you should be fine.
You may want to try out the calculator with the actual information of your system - like any powered USB items, etc.