Apparently the 'high' textures are only available if your graphics card has 512 MB of memory. (
Source)
I've played the demo a little. The first impression was actually nauseating, but installing the latest graphics drivers improved the frame rate enough to cure that. Even so, just around 30 fps, which is no more than tolerable, I guess.

Well, I already pre-ordered the game a couple of weeks ago, so I'll have to manage...