Forced ATI 4890 to normal 3D clock speed, ie 870MHz, (using ATI tray tools) and, surprise, the game is great, still stutters but really another experience compared to previously stuck at 500MHz.
50 fps above sea
23-25 above land
in the english campaign missions
So, ATI owners try this
running game at 1680x1050, no AA, everything high or more, incl grass/forest=4