Here is how to change the MCLK frequency of a 3Dlabs' Reference Permedia
board under Windows 95.

Invoke REGEDIT using RUN from the Start button ...
  My Computer
    HKEY_LOCAL_MACHINE
      System
        CurrentControlSet
          Services
            Class
              Display
                000X (where X is the following Default containing
                      references to Permedia)
                  DEFAULT
                    SystemClock (Value usually is 3C hexidecimal, reset to 32)

  Typical Values
    $32 = 50 MHz    used only for early prototype boards
    $3C = 60 MHz    this is the driver default, this will work on boards
                    fitted with a mezzanine
    $50 = 80 MHz    rock solid setting for all recent boards without
                    mezzanines fitted
    $53 = 83 MHz    optimum setting for most recent boards without
                    mezzanines fitted
    $5A = 90 MHz    fast setting, video noise present.
                    New 'LC' boards have no noise at this setting
    $64 = 100 MHz   future setting ...?
                    As yet no board has been run without considerable
                    video noise (although designs to reach this are in
                    progress)

Use the Display Control Panel to modify any display setting (resolution,
color depth) dynamically, then restore old setting.  This will bring the
new clock rate into effect.

Updated, October 30, 1996 - Frederick Shaul