After the game running smoothly, as I entered the 1st Act Battlefield, everything slowed down to almost unplayable. When I went into the dungeons, the movement rate went back to almost normal, but I have the game set at the 1024x768 in order to keep the speed up. I have an i7 quad-core and 3gigs of DDR3 ram and ATI HD4850 video card with 1gig of memory, so I'm surprised I'm having this kind of problem. Perhaps the Vista 64bit is causing the problem, but as I started the 2nd act, the game is slow even at 1024x768. If I go to the 800x600 route, everything gets too big to fit onscreen. Yikes.
DD ran smooth as silk for the entire game for me too.