It is easy to change configurations while you are flying or taxiing. First, create a few configurations with appropriate scenery settings (e.g. mesh resolution, mesh complexity, scenery complexity, AI Traffic complexity, cloud draw distance, frame rate, etc.) and save them using Options > Settings > Save. For example, on my previous computer (Vista 32-bit) I had saved configurations for Taxi, Takeoff/Landing, Low Altitude Cruise, and High Altitude Cruise. (My current Windows 7 64-bit computer has sufficient resources to use the same config file for all phases of flight.) When you want to change, simply use Options > Settings > Load to select the appropriate file "on the fly".