How to set screen resolution to 1366x768

Question

I recently installed Ubuntu 14.04, this is my first time using Linux and I'm having trouble with the settings of my screen and my GPU.

I have a S13A100N Samsung monitor with native resolution of 1366 x 768 at 60 Hz and a GeForce GTS 450 graphics card.

After searching different solutions, I installed the video drivers using this method http://www.howopensource.com/2012/10/install-nvidia-geforce-driver-in-ubuntu-12-10-12-04-using-ppa/ and it's now using NVIDIA legacy binary driver - version 304.131 from nvidia-340 (proprietary)

When using xrandr to add a new resolution, it fails at --newmode.

    adrianrojas@Hyperborea:~$ cvt 1366 768
    # 1368x768 59.88 Hz (CVT) hsync: 47.79 kHz; pclk: 85.25 MHz
    Modeline "1368x768_60.00"   85.25  1368 1440 1576 1784  768 771 781 798 -hsync +vsync
    adrianrojas@Hyperborea:~$ xrandr
    Screen 0: minimum 8 x 8, current 1024 x 768, maximum 16384 x 16384
    DVI-I-0 disconnected (normal left inverted right x axis y axis)
    DVI-I-1 connected primary 1024x768+0+0 (normal left inverted right x axis y axis) 338mm x 270mm
       1024x768       60.0*+   75.0     70.1  
       1280x1024      75.0     60.0  
       800x600        75.0     72.2     60.3     56.2  
       640x480        75.0     72.8     59.9  
    DVI-I-2 disconnected (normal left inverted right x axis y axis)
    HDMI-0 disconnected (normal left inverted right x axis y axis)
    DVI-I-3 disconnected (normal left inverted right x axis y axis)
      1368x768_60.00 (0x2c1)   85.2MHz
            h: width  1368 start 1440 end 1576 total 1784 skew    0 clock   47.8KHz
            v: height  768 start  771 end  781 total  798           clock   59.9Hz
adrianrojas@Hyperborea:~$ xrandr --newmode "1368x768_60.00"   85.25  1368 1440 1576 1784  768 771 781 798 -hsync +vsync
X Error of failed request:  BadName (named color or font does not exist)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  16 (RRCreateMode)
  Serial number of failed request:  31
  Current serial number in output stream:  31
adrianrojas@Hyperborea:~$ $ xrandr --addmode DVI-I-1 1368x768_60.00
$: command not found

I'd really appreciate the help, I'm having problems understanding and using Ubuntu at the moment. Thanks.