Here's the route I took:
1. Got the desktop 6.2 drivers
2. Got D-Force MobileMod from http://downloads.guru3d.com/download.php?det=602
3. Install the 6.2 package. It will fail with something like "unrecognized video card".
4. Run mobilemod. Note that when it looks for the inf file to modify, there's a choice at the bottom of the dialog for Win2K or WinXP drivers - pick the appropriate one and find the inf file which will start with C2 or Cx.
5. After mobilemod is done, find the relevant inf file in \ATI\Support\... and right-click on it, and select install. If the installer prompts to search, say that you'll designate the location and that you have a disk (this can be done from the device manager for the display adapter as well).
6. You'll get a loooong list of video cards. Find the x1400 and select it.
7. When it's done, reboot and you should be on your way. You can verify that the new drivers are installed from the device manager or from the ATI CCC.