Weird I got the XFX HD 5770 on my hackintosh and I had no problems with this card at all on 10.6.7 and 10.6.8. Try these out and see if this helps.
1. Try reinstalling latest version of Chimera either from Muiltibeast or just download it by itself.
2. Still didn't help? Try putting these strings in your plist file and see if this helps:
<key>AtiFb</key>
<string>Vervet</string>
<key>Graphics Mode</key>
<string>1920x1080x32</string> <--- (this is for your boot screen resolution)
<key>GraphicsEnabler</key>
<string>Yes</string> <--- (if you still have an issue with graphics not working properly try "no" instead of "yes")
Let me know how that works for you