Maybe it's driver optimization. What's your monitor as well?
I'm using a 27" ASUS 144hz monitor with a 3gig AMD Sapphire 7950 - 3.2ghz quadcore AMD cpu, 4 gigs o' ram, 7200 HDD, Vista 64bit. And it all runs smooth and fast for me. There are some areas where I have seen a bit of stuttering and clipping but I can't seem to pin it down. Overall I'm pretty impressed with the performance for the current state of the game.
