Thanks go to Burkhard Renk for this tip!

While FS9 typically uses between 20 and 50 MB of video memory, FSX has been observed using between 110 and 580 MB!  This is mainly due to texture formats, the pure number of different textures for the autogen, etc. Graphics adapters today have 128, 256 or 512MB typically - plus use system memory in case they need more.

If this happens as a peak, there is no problem with it, but if it happens constantly, the difference between the current usage and the on graphics card hardware has to be transferred over the system bus PER FRAME.

So let as assume you are proud of a high end graphics adapter with 256MB, but run FSX with settings to use 380 of them, and set the target frame rate to 25. Doing so you order 124 MB to be transferred per frame, or 3.1 GB per second over the system bus (that can do only on average 4.2 GB peak rate!, so 2-3 average rate). Obviously, that will not do well.

So, if you have a graphics adapter with less than 512 MB, (some addons will be able to flood that too I'm sure ), I strongly recommend that you use the little utility MEM_STATUS from http://www.nuclearplayground.com/NuclearPlayground/MemStatus/ and monitor the usage of video memory.

On your FSX install, go to a typical situation you want to perform smooth, (That isn't over the middle of the ocean), put ALL sliders to the far left, switch all options (shadows, etc off), filtering to none, etc... and vary the screen resolution. Next increase slowly those sliders that you consider important, like AI traffic density, global texture resolution, scenery density, and you will see the video memory usage climbing. You will very quickly get a feeling of what costs a lot, and what is cheap.

Do so, increasing to your importance list, until you reach
on 128MB adapters: 118 MB
on 256MB adapters: 240 MB
on 512MB adapters: 490 MB

Doing so, you will find a setting where you get what your hardware can do, and if you avoid the memory hungry ones, it will be looking good, and running smooth.

Here is a composite image of MemStatus illustrating the "Memory Load" of FS9, FSX Overloaded, and FSX Balanced: