I think the HD textures seems to be the cause, but i don't really know. I have a 460GTX card and it runs the game really well providing i don't put shadows on ultra. But since i've installed the HD textures (may not be related), many graphics options seem to be unchangeable, including shadows and textures, which are stuck on ultra. I could try changing settings in skyrimprefs.ini but i don't really know where to look. I'd really like to get the shadow settings down to high or some other reasonable setting, because there is barely a change and it seems to have a massive impact on smoothness.