The specific video format that an SGI displays is controlled by the command
/usr/gfx/setmon.
On all machines except the Infinite Reality, the formats are restricted to those provided by Silicon Graphics. The common formats are listed in the setmon man page. All of the formats available are in a subdirectory of /usr/gfx/ucode; the man page tells which exact directory, which depends on the graphics hardware.
For example:
/usr/gfx/setmon -n 75hzwill change the display to be 1280x1024 at 75Hz (if the hardware supports this format).