Try switching off tree crown,or the tree foliage,that should speed things up quite alot although the trees will bald. And Lighting quality is another major killer,try to play around with that variable. And always use the antialiasing/aniso settings in the game options instead of forcing it thru drivers,that will be counterproductive.
Massive chain explosions usually cause stutter,regardless of machine spec,due to the physics calculations i presume.
That's all i can think of at the moment.
Buy this game,if u even remotely enjoyed Jagged Alliance,XCom or Fallout Tactics. This game is more than a worthy predecessor to the "Kings" of the turn based genre.