Do you run with all of the options on? Do you run on NonPhotoRealistic fields? (The PhotoRealistic fields require less processing power as indicated by the greater frame-rate). What is your frame-rate?
I have a 2.4 Ghz Celeron with 2.0 GB RAM, and a PCI Nvidia
6200. I can't even get a decent framerate on 4.0 on the NonPhoto fields unless I turn the options down a little bit. No clouds no shadows, low foliage density.