See Kano's Scriptpage for Kanotix / Knoppix
. If you scroll past halfway down the page, you will find scripts for installing nVidia and ATI drivers.
These scripts are apparently included with the Overclockix
I can't comment on whether these scripts work, since I'm still using a Matrox G450 (which I think is still the best video card for Linux; it's great with Quake3).
BTW, if you are doing a hard-drive install, I strongly recommend Kanotix
instead of Knoppix. Knoppix was not intended to be installed to the hard-drive; its mix of Debian Sarge and Sid packages can cause problems with apt-get. Kanotix uses Sid packages only; I believe that Overclockix and Mepis do as well.