Ubuntu – Cannot get resolution of 1280×800 on the “SiS M760GX” video card

This is a near-perfect solution for me to get a better resolution than 640×480, but is still not quite right!

I have a an Acer Aspire 5000 with "SiS M760GX" video card (lspci -v returns Silicon Integrated Systems [SiS] 661/741/760 PCI/AGP or 662/761Gx PCIE VGA Display Adapter (prog-if 00 [VGA controller])) and using this xorg.conf file shown below I can now get resolutions of 1280×768, 1024×768, 800×600, or 640×480, instead of just 640×480, which was waaaaay too low of a resolution. However, my screen's native resolution is 1280×800, so it still looks a little off. ANY IDEA HOW I CAN GET 1280X800 TO WORK?

Note that I'm running Xubuntu 16.04.3 64-bit. The Acer Aspire 5000 is from approximately 2005.

Here's my /etc/X11/xorg.conf file (Originally posted here by javier-ejsf [post #10]! – https://ubuntuforums.org/showthread.php?t=2215422):

Section "Device"
  Identifier "Generic Video Card"
    VendorName  "Silicon Integrated Systems [SiS]"
        BoardName   "771/671 PCIE VGA Display Adapter"
    Busid "PCI:1:0:0"
    Driver "vesa"
    Screen 0
        Option "UseFBDev" "true"
        Option "DPMS"
        Option "ShadowFB"
        Option "MaxXFBMem"
        VideoRam 262016
        Option "RenderAccel" "true"
        Option "AllowGLXWithComposite" "true"
        Option "backingstore" "true"
        Option "AddARGBGLXVisuals" "True"

EndSection

Section "Monitor"
    Identifier    "Configured Monitor"
    Vendorname    "Generic LCD Display"
    Modelname    "LCD Panel 1280x800"
    HorizSync 20-107
        VertRefresh 50-185
  modeline  "800x600@56" 36.0 800 824 896 1024 600 601 603 625 +hsync +vsync
  modeline  "800x600@60" 40.0 800 840 968 1056 600 601 605 628 +hsync +vsync
  modeline  "1280x768@60" 80.14 1280 1344 1480 1680 768 769 772 795 -hsync +vsync
  modeline  "1280x720@60" 74.48 1280 1336 1472 1664 720 721 724 746 -hsync +vsync
  modeline  "1280x800@60" 83.46 1280 1344 1480 1680 800 801 804 828 -hsync +vsync
    Gamma    1.0
EndSection

Section "Screen"
    Identifier    "Default Screen"
    Monitor        "Configured Monitor"
    Device        "Configured Video Device"
    Defaultdepth    24
    SubSection "Display"
        Depth    24
        Virtual    1280    768
        Modes        "1280x768@60"    "1280x720@60"    "800x600@60"    "1280x800@60"    "800x600@56"
    EndSubSection
EndSection

Section "Module"
    Load "dri"
    Load "dbe" # Double-Buffering Extension
    Load "v4l" # Video for Linux
    Load "extmod"
    Load "type1"
    Load "freetype"
    Load "glx" # 3D layer
    Load "GLcore"
    Load "i2c"
    Load "bitmap"
    Load "ddc"
    Load "int10"
    Load "vbe"
    Load "speedo"
    Load "record"
EndSection

Section "DRI"
        Mode 0666
EndSection

What' I've already tried:
If I try changing "Virtual 1280 768" to "Virtual 1280 800" all it does is make my screen vertically extend beyond the edges of the monitor, so by moving the mouse to the top or the bottom it will pan the screen up or down to see the extra 32 pixels. This is NOT what I want.