Here's what worked for me -- boot knoppix from CD (not an HD install) and copy the XF86Config-4 file from /etc/X11/ to the same directory on your HD install. If the CD boots OK, I think you can use the XF86Config-4 that it creates.

do a "cp -p XF86Config-4 XF86Config-4.bak" first, to save the original before you overwrite it on HD.

(Actually, I didn't just change the video card; I copied an HD install to a partition on another system, so video and
everything was different.)

Hope this helps!