I highly recommend nHancer and can suggest the following (anything not mentioned should be left at default, usually "off")
Multisampling - 16Q (nicer antialiasing, but you get no sun bloom so not as nice sunrises/sunsets. If you want the sun to look "right", use 4x4 instead.)
Supertransparency box checked
Anisotropic Filtering - 16x
V Sync set to App
Texture Filtering - High Quality
Negative LOD - Clamp
Compatibility - EA Strategy Games
This is on an nVidia 9800GT which is really the old 8800 re-badged, you don't mention which card you have but this should be a good step in the right direction. Hope it helps!