on my system(which is fairly similar to yours, running vista 64bit), when in DX9 mode, game runs fine on high settings @ 1440*900. when in DX10 mode, i have similar problems as you do when i'm @ med settings. since the game really isn't in full DX10 except at very high settings, try right clicking the icon and going to "Run in DX9". might be what you're looking for as far as performance, might not.