I usually run FSX in windowed mode, as I use my second monitor to display other stuff at the same time, but it wasn't until recently I found that if I hid the taskbar, my frame rates increased dramatically.
Here are some screenshots to illustrate:
First, normal windowed with desktop taskbar showing (not visible in screenshot)

Same view with taskbar hidden

No AI enabled, and no frame rate limit set for these tests.
Screenshots reduced to 800px wide, frame rate display enlarged for clarity.
Tried the same with FS9, but without any noticable difference.
Running Win7, not tried this with Vista or XP.
YMMV





