You can force Knoppix to use the "nv" driver by using the "cheatcode" xmodule=nv. If that doesn't work, try using xmodule=vesa to force it to use the generic VESA driver. You should also be able to specify your screen resolution by using the "cheatcode" screen=1280x1024 (or whatever), although I have never been able to get Knoppix to go higher than 1024x768. (Info from the cheatcodes doc and the FAQ.)