For Nvidia users right click your desktop and click properties to bring up the display control window. Click the settings tab at the top and find the button marked "advanced." Click on the tab marked with the name of your card i.e. GeForce 6600. Anti-Aliasing and Anisotropic Filter settings ore under the tab marked Peformance & Quality.
Play arround with the different settings to see which suits your system.
I would also try turning the shadows down with mirrors and heat blur off. You just need to find the sweet spot for your system.
I run at.
Terrain: High
Textures: High
Effects: High
Water: High
Vis Range: Medium
Preload: 100k
Res: 1280x960
Shadows: Full
Lights: All
Haze: Adv
Heat Blur: On
Civil Traffic: On
And My System...
Pentium 3.2E
1.5Gig DDR400
GeForce 6800 Ultra
Epox EP-4PGAI Motherboard