I've taken a look at my Xorg.0.log file and, among other things, it has:-
(--) GLINT(0): FIFO Size is 256 DWORDS (==) GLINT(0): Min pixel clock is 16 MHz (--) GLINT(0): Max pixel clock is 110 MHz (II) GLINT(0): LGFlatron: Using hsync range of 30.00-107.00 kHz (II) GLINT(0): LGFlatron: Using vrefresh range of 50.00-200.00 Hz (II) GLINT(0): Clock range: 16.25 to 110.00 MHz (II) GLINT(0): Not using default mode "320x175" (bad mode clock/interlace/doublescan) (II) GLINT(0): Not using default mode "320x200" (bad mode clock/interlace/doublescan) (II) GLINT(0): Not using default mode "360x200" (bad mode clock/interlace/doublescan) (II) GLINT(0): Not using default mode "320x240" (bad mode clock/interlace/doublescan) (II) GLINT(0): Not using default mode "320x240" (bad mode clock/interlace/doublescan) (II) GLINT(0): Not using default mode "320x240" (bad mode clock/interlace/doublescan) (II) GLINT(0): Not using default mode "320x240" (bad mode clock/interlace/doublescan) (II) GLINT(0): Not using default mode "400x300" (bad mode clock/interlace/doublescan) (II) GLINT(0): Not using default mode "400x300" (bad mode clock/interlace/doublescan) ... ... ... (II) GLINT(0): Not using default mode "1600x1200" (bad mode clock/interlace/doublescan) ... ... ... (II) GLINT(0): Not using mode "1600x1200" (no mode of this name) ... ... ... (--) GLINT(0): Virtual size is 1280x1024 (pitch 1280) (**) GLINT(0): *Default mode "1280x1024": 108.0 MHz, 64.0 kHz, 60.0 Hz (II) GLINT(0): Modeline "1280x1024" 108.00 1280 1328 1440 1688 1024 1025 1028 1066 +hsync +vsync (**) GLINT(0): *Default mode "1024x768": 94.5 MHz, 68.7 kHz, 85.0 Hz (II) GLINT(0): Modeline "1024x768" 94.50 1024 1072 1168 1376 768 769 772 808 +hsync +vsync (**) GLINT(0): Default mode "1280x960": 108.0 MHz, 60.0 kHz, 60.0 Hz (II) GLINT(0): Modeline "1280x960" 108.00 1280 1376 1488 1800 960 961 964 1000 +hsync +vsync (**) GLINT(0): Default mode "1152x864": 108.0 MHz, 67.5 kHz, 75.0 Hz (II) GLINT(0): Modeline "1152x864" 108.00 1152 1216 1344 1600 864 865 868 900 +hsync +vsync (**) GLINT(0): Default mode "1152x768": 65.0 MHz, 44.2 kHz, 54.8 Hz
The 1280x1024 line is the highest resolution one that appears with that "Modeline" start, does that mean that it's the highest resolution that Xorg thinks is possible?