dapper system freeze

Twice since I upgraded to dapper two or three weeks ago I've had it completely freeze on me. The computer just completely stops responding - mouse, keyboard, ctrl-alt-bkspce, screen is frozen, everything. I have to kill it at the power switch. It causes me to lose work.

Since it's frozen, I can't do anything to query the system and find out what's happening. I would very much like to get to the bottom of this and at least file a good bug report or find an existing one. There must be some log files I should look at after rebooting the computer?

I'm not using the restricted nvidia or ati drivers (I don't think). I think I'm using the open source xserver-xorg-driver-ati package.

There's a bunch of system freezes related to ati cards in malone, but none of them sound exactly like my problem, and I don't know how to verify if I'm experiencing the same thing. My problem doesn't seem to occur when logging in or logging out or restarting X or switching to dual monitor, or running opengl apps, nor is it very frequent, it just occurred twice, when working normally with normal GTK apps, nothing special.

lshw for the machine:
https://wiki.ubuntu.com/SeanHammond?action=AttachFile&do=get&target=marley-lshw.html

xorg.conf for the machine (which was in /etc/X11/ and had a modified date of 24 January, there is another xorg.conf in /usr/share/xresprobe which is more recent-23 Mar-but I dunno what it's for):

# /etc/X11/xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the /etc/X11/xorg.conf manual page.
# (Type "man /etc/X11/xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
# sudo dpkg-reconfigure -phigh xserver-xorg

Section "Files"
 FontPath "/usr/share/X11/fonts/misc"
 FontPath "/usr/share/X11/fonts/cyrillic"
 FontPath "/usr/share/X11/fonts/100dpi/:unscaled"
 FontPath "/usr/share/X11/fonts/75dpi/:unscaled"
 FontPath "/usr/share/X11/fonts/Type1"
 FontPath "/usr/share/X11/fonts/CID"
 FontPath "/usr/share/X11/fonts/100dpi"
 FontPath "/usr/share/X11/fonts/75dpi"
        # paths to defoma fonts
 FontPath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
 FontPath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/CID"
EndSection

Section "Module"
 Load "GLcore"
 Load "bitmap"
 Load "ddc"
 Load "dri"
 Load "extmod"
 Load "freetype"
 Load "glx"
 Load "int10"
 Load "type1"
 Load "vbe"
EndSection

Section "InputDevice"
 Identifier "Generic Keyboard"
 Driver "kbd"
 Option "CoreKeyboard"
 Option "XkbRules" "xorg"
 Option "XkbModel" "pc105"
 Option "XkbLayout" "gb"
EndSection

Section "InputDevice"
 Identifier "Configured Mouse"
 Driver "mouse"
 Option "CorePointer"
 Option "Device" "/dev/input/mice"
 Option "Protocol" "ImPS/2"
 Option "Emulate3Buttons" "true"
 Option "ZAxisMapping" "4 5"
EndSection

Section "Device"
 Identifier "ATI Technologies, Inc. Radeon 7500 (RV200 QW)"
 Driver "ati"
 BusID "PCI:1:0:0"
EndSection

Section "Monitor"
 Identifier "HP L1740"
 Option "DPMS"
EndSection

Section "Screen"
 Identifier "Default Screen"
 Device "ATI Technologies, Inc. Radeon 7500 (RV200 QW)"
 Monitor "HP L1740"
 DefaultDepth 24
 SubSection "Display"
  Depth 1
  Modes "1280x1024" "1024x768" "800x600" "640x480"
 EndSubSection
 SubSection "Display"
  Depth 4
  Modes "1280x1024" "1024x768" "800x600" "640x480"
 EndSubSection
 SubSection "Display"
  Depth 8
  Modes "1280x1024" "1024x768" "800x600" "640x480"
 EndSubSection
 SubSection "Display"
  Depth 15
  Modes "1280x1024" "1024x768" "800x600" "640x480"
 EndSubSection
 SubSection "Display"
  Depth 16
  Modes "1280x1024" "1024x768" "800x600" "640x480"
 EndSubSection
 SubSection "Display"
  Depth 24
  Modes "1280x1024" "1024x768" "800x600" "640x480"
 EndSubSection
EndSection

Section "ServerLayout"
 Identifier "Default Layout"
 Screen "Default Screen"
 InputDevice "Generic Keyboard"
 InputDevice "Configured Mouse"
EndSection

Section "DRI"
 Mode 0666
EndSection