In order to maximize X-Plane’s framerate, you need to start with the lowest settings and work your way up. Can you start with the highest settings and work your way down? No! Here’s the problem:
X-Plane’s framerate is determined by the slowest part of the system…whether it’s your CPU, AGP bus, or any of the parts of your graphics card (pixel shaders, lack of VRAM, lack of internal card memory bandwidth). Whatever the problem is, the graphics are drawn in an assembly line, and the rest of the hardware sits bored while the slowest part finishes its backlog.
So…why not start with the highest settings and turn them down? The answer is that when the framerate is slowed down by component A, changes in the load on component B won’t show as a difference of framerate, because component B isn’t the slowest. So as you turn down settings, you’re not seeing the real effect. (The safe thing to do would be: turn B back to its old setting, then retry.)
What I recommend is: start on the lowest settings possible – you’ll see a framerate that’s about as high as your system can go (limited by the FM and the speed the card can turn a frame around). Then turn settings up, one at a time, by one notch. Each time you see a fps hit, back off one setting and continue somewhere else. This way you won’t load up one part of the system so heavily that you are limiting the speed despite everything else.