my 2 pence,
Seems that Vista/DX10 is too much of a performance hog. With the system in my sig, COH-OF runs like a dog on Vista, but silky smooth in XP. Have all the updated drivers and have the settings exactly the same (obviously apart from the DX setting, plus I have my resolution in Vista set to 1024 instead of 1280 in XP!).
I can tell the difference in image quality, and the little pieces of litter are quite nice, but it runs soooo slow on Vista its really not worth it.