i run assassins creed on highest setting and a resulotion of 1024x768 at 75 Hz.. upgrading to 4GB DDR3 is quite well.. and i dont think i should upgrade for vista with my current PC (unless getting a new processor also, or overclocking it by buying a new motherboard.)
oh, and do you think IV will have better graphics? at the pool (not the ones you jump into it and its full of water!) i saw that they prefer something else.. still better replay will be good. since we wont have to buy software such as Fraps, GameCam etc..