Okay, here's another data point (I have the UK version):
I get the hard disk swapping and, what really bothers me: blurry textures, and trees keep going all funny and less detailed.
I have all the settings maxed out (see my system below). I haven't measured the framerate but it seems smooth.
I tried turning the texture detail down from "very high" to "high", and I've also tried reducing the visibility from 10km to 7km. That helped a little bit, I think, but did not fix the problem. It seems the blurry textures are worse at first and after about ten minutes of playing they get better, but still the odd building is blurry.
If I stand still for a few seconds the textures eventually load and the trees become detailed. If I spin round 180 degrees the disk starts up, buildings and cars go blurry, and trees become less detailed. Then after a few seconds textures and trees resolve to full detail. If I spin round 180 degrees back to my original position it starts again: everything is blurry.
If it wasn't for the trees I'd say it was a texture caching problem, but more than that must be going on. Perhaps it's a wierd level of detail bug, or perhaps a memory issue causes the game to switch models to a lower LOD.
I didn't see any of these problems with the demo.
Vista is very new; x64 is supposedly less compatible; and the Nvidia drivers for Vista are new (I currently have version 100.64) -- so all kinds of potential sources of problems there...