I have a Compaq Presario v2413us running Mandriva 2006 x64. The best color depth is 24 bit (NOT 32 bit), but 1280x768 is definitely possible. Here is my xorg.conf, hope it helps:
# File generated by XFdrake.
# **********************************************************************
# Refer to the XF86Config man page for details about the format of
# this file.
# **********************************************************************
Section "Files"
# Multiple FontPath entries are allowed (they are concatenated together)
# By default, Mandrake 6.0 and later now use a font server independent of
# the X server to render fonts.
FontPath "unix/:-1"
EndSection
Section "ServerFlags"
#DontZap # disable <Crtl><Alt><BS> (server abort)
AllowMouseOpenFail # allows the server to start up even if the mouse does no
t work
#DontZoom # disable <Crtl><Alt><KP_+>/<KP_-> (resolution switching)
EndSection
Section "Module"
Load "dbe" # Double-Buffering Extension
Load "v4l" # Video for Linux
Load "extmod"
Load "type1"
Load "freetype"
Load "synaptics"
Load "glx" # 3D layer
Load "dri" # direct rendering
EndSection
Section "InputDevice"
Identifier "Keyboard1"
Driver "keyboard"
Option "XkbModel" "pc105"
Option "XkbLayout" "en_US"
Option "XkbOptions" "compose:rwin"
EndSection
Section "InputDevice"
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "ExplorerPS/2"
Option "Device" "/dev/mouse"
Option "ZAxisMapping" "6 7"
EndSection
Section "InputDevice"
Identifier "SynapticsMouse1"
Driver "synaptics"
Option "Protocol" "auto-dev"
Option "Device" "/dev/input/mice"
Option "MinSpeed" "0.09"
Option "MaxSpeed" "0.18"
Option "BottomEdge" "4200"
Option "SHMConfig" "on"
Option "FingerLow" "25"
Option "LeftEdge" "1700"
Option "MaxTapMove" "220"
Option "MaxTapTime" "180"
Option "FingerHigh" "30"
Option "VertScrollDelta" "100"
Option "TopEdge" "1700"
Option "AccelFactor" "0.0015"
Option "RightEdge" "5300"
EndSection
Section "Monitor"
Identifier "monitor1"
VendorName "Plug'n Play"
ModelName ""
HorizSync 56-56
VertRefresh 72-72
# Monitor preferred modeline (60.0 Hz vsync, 49.4 kHz hsync, ratio 15/9)
ModeLine "1280x768" 71.1 1280 1296 1408 1440 768 769 772 823 -hsync -vsync
# TV fullscreen mode or DVD fullscreen output.
# 768x576 @ 79 Hz, 50 kHz hsync
ModeLine "768x576" 50.00 768 832 846 1000 576 590 595 630
# 768x576 @ 100 Hz, 61.6 kHz hsync
ModeLine "768x576" 63.07 768 800 960 1024 576 578 590 616
# modeline generated by gtf(1) [handled by XFdrake]
ModeLine "1280x768_120" 173.82 1280 1376 1520 1760 768 769 772 823 -HSyn
c +Vsync
# modeline generated by gtf(1) [handled by XFdrake]
ModeLine "1280x768_100" 141.96 1280 1376 1512 1744 768 769 772 814 -HSyn
c +Vsync
# modeline generated by gtf(1) [handled by XFdrake]
ModeLine "1280x768_85" 118.53 1280 1368 1504 1728 768 769 772 807 -HSync
+Vsync
# modeline generated by gtf(1) [handled by XFdrake]
ModeLine "1280x768_75" 102.98 1280 1360 1496 1712 768 769 772 802 -HSync
+Vsync
# modeline generated by gtf(1) [handled by XFdrake]
ModeLine "1280x768_60" 80.14 1280 1344 1480 1680 768 769 772 795 -HSync
+Vsync
# modeline generated by gtf(1) [handled by XFdrake]
ModeLine "1280x768_50" 65.18 1280 1336 1464 1648 768 769 772 791 -HSync
+Vsync
EndSection
Section "Device"
Identifier "device1"
VendorName "ATI"
BoardName "ATI Radeon (fglrx)"
Driver "ati"
Option "DPMS"
EndSection
Section "Screen"
Identifier "screen1"
Device "device1"
Monitor "monitor1"
DefaultColorDepth 24
Subsection "Display"
Depth 8
Virtual 1280 768
EndSubsection
Subsection "Display"
Depth 15
Virtual 1280 768
EndSubsection
Subsection "Display"
Depth 16
Virtual 1280 768
EndSubsection
Subsection "Display"
Depth 24
Virtual 1280 768
EndSubsection
EndSection
Section "ServerLayout"
Identifier "layout1"
InputDevice "Keyboard1" "CoreKeyboard"
InputDevice "Mouse1" "CorePointer"
InputDevice "SynapticsMouse1" "AlwaysCore"
Screen "screen1"
# Option "StandbyTime" "1"
# Option "SuspendTime" "2"
# Option "OffTime" "2"
EndSection
