pg@pipoTower: ~/tmp$ xrandr
Screen 0: minimum 8 x 8, current 2720 x 1080, maximum 16384 x 16384
DVI-I-0 disconnected (normal left inverted right x axis y axis)
DVI-I-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 531mm x 298mm
1920x1080      60.0*+
1680x1050      60.0
1600x900       60.0
1280x1024      75.0     60.0
1280x800       59.8
1280x720       60.0
1024x768       75.0     60.0
800x600        75.0     60.3
640x480        75.0     59.9
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-0 disconnected (normal left inverted right x axis y axis)
DVI-D-0 connected 800x600+1920+0 (normal left inverted right x axis y axis) 0mm x 0mm
800x600        60.3*+
DP-1 disconnected (normal left inverted right x axis y axis)

Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "BenQ GL2450H"
    HorizSync       30.0 - 83.0
    VertRefresh     50.0 - 76.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 660"
    Option         "ModeValidation" "NoMaxPClkCheck"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "Stereo" "0"
    Option         "nvidiaXineramaInfoOrder" "DFP-0"
    Option         "metamodes" "DVI-I-1: nvidia-auto-select +0+0, DVI-D-0: 1024x768_60 +1920+0"
    Option         "SLI" "Off"
    Option         "MultiGPU" "Off"
    Option         "BaseMosaic" "off"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

pg@pipoTower: ~/tmp$ cvt 1024 768 60
# 1024x768 59.92 Hz (CVT 0.79M3) hsync: 47.82 kHz; pclk: 63.50 MHz
Modeline "1024x768_60.00"   63.50  1024 1072 1176 1328  768 771 775 798 -hsync +vsync
pg@pipoTower: ~/tmp$ xrandr --newmode "1024x768_60.00"   63.50  1024 1072 1176 1328  768 771 775 798 -hsync +vsync
pg@pipoTower: ~/tmp$ xrandr --addmode DVI-D-0 1024x768_60.00
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:  41
Current serial number in output stream:  42