If you tweak the configuration files in CVarGroups by copying and pasting the "very high" settings (1st paragraph) IN PLACE of the "high" settings (last paragraph) the game will load the highest possible settings even though the drop-down menus display "high." The difference between "high" settings and the tweaked settings is immense: shadows are deeper, more realistic; the leaves have better reflective properties, better textures; the colours are better; and the level of detail is simply stunning.

With these settings I'm running the game between 15-25 FPS at 1440x900 and (wait for it) 8x AA, and it looks PERFECT. Best of all, this is in XP. So I'm happy. I have XP and I'm playing the game at settings higher than DX9 allows (strictly speaking). Give this a try if your rig can handle it.