Try right-clicking on your desktop, select "Properties" and go to "settings-->Advanced" to get at your monitor settings.
Most video card drivers have an option to enable/disabled vertical syncing. If vertical sync is enabled, your graphics card won't send frames to your monitor any faster than the monitor can display them, so your framerate will always top out at whatever refresh rate you've set for the monitor.
I have a Voodoo 3 and when vsync is enabled, I get a rock-solid 60FPS during the Nali Castle flyby (since my monitors refresh rate is 60Mhs). When vsync is disabled that figure goes up and varies between 80-90 FPS.
I noticed some small graphics problems when VSync is disabled. When I go into an area with a flickering light source, especially, I get faint, flickery horizontal banding across my monitor. I'm assuming it's because my cards refresh rate is not in sync with the monitor but I'm too lazy to fix it.
--Nut