Well, i do not understand it, but i have returned to my old 285, and the frame rates are better now.
I play at 1920x1200 with high level of AA/AF.
I have been reading the technical details of the 480 vs the 285:
Maybe the higuer number of texture units in the 285 could explain the performance drop with the settings that i use.
I really dunno how to explain this.