Have you tried using the original configuration detected by knoppix(the first XF86Config-4) but change the lines :

Modes "1024x768" "800x600" "640x480"

to Modes "800x600" "640x480"

Seems that the driver does support your card at the highest resolution so I can't see why it cannot be done for 800x600, may be all you need is another ModeLine.