Vista or xp?
weird thing is, it runs nicely in DX10 for me if you disable streaming (e_stream=0, r_texturesstreaming=0, etc) and disable superfetch, feels nice and smooth, plus it loads a LOT faster in DX10 than DX9, for some reason during level loading theres a huge pause in DX9 mode whereas DX10 mode simply says 'press any key to continue'
You'll notice that most levels require 500-600mb less memory on DX10. What does e_stream do, first time I've seen that command!