nikita@nikita:~$ xrandr
Screen 0: minimum 8 x 8, current 1024 x 768, maximum 16384 x 16384
VGA-0 connected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   1024x768      60.00*+
   1600x900      59.82  
   1400x900      59.88  
   1368x768      59.88    59.85  
   1360x768      59.96    59.80  
   1280x800      59.91    59.81  
   1280x720      59.86    59.74  
   1152x864      60.00  
   1024x576      59.90    59.82  
   960x540       59.82    59.63  
   864x486       59.92    59.57  
   800x600       72.19    60.32    56.25  
   800x450       59.82  
   700x450       59.88  
   684x384       59.88    59.85  
   680x384       59.96    59.80  
   640x480       59.94  
   640x400       59.98    59.88  
   640x360       59.86    59.83  
   512x384       60.00  
   512x288       60.00    59.92  
   480x270       59.82    59.63  
   432x243       59.92    59.57  
   400x300       72.19  
   320x240       60.05  
DVI-D-0 disconnected (normal left inverted right x axis y axis)
HDMI-0 disconnected (normal left inverted right x axis y axis)
DVI-D-1 disconnected (normal left inverted right x axis y axis)

nikita@nikita:~$ cvt 1280 1024 75
# 1280x1024 74.90 Hz (CVT 1.31M4) hsync: 80.30 kHz; pclk: 138.75 MHz
Modeline "1280x1024_75.00"  138.75  1280 1368 1504 1728  1024 1027 1034 1072 -hsync +vsync

nikita@nikita:~$ xrandr --newmode "1280x1024_75.00"  138.75  1280 1368 1504 1728  1024 1027 1034 1072 -hsync +vsync

nikita@nikita:~$ xrandr --addmode VGA-0 "1280x1024_75.00"
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  18 (RRAddOutputMode)
  Serial number of failed request:  35
  Current serial number in output stream:  36