Does knoppix run your video OK if you boot from a cd?

Check out the boot parameter for refresh rate and play around with it, once you get your screen resolution and refresh rate the way you want it mount your hd read/write and save the created /etc/X11/XF86Config-4 file to your hd and then boot your hd install.

That is the easiest way to "fix" your video with knoppix, the "pure" debian way is to run this command:
dpkg-reconfigure -pmedium xserver-xfree86
and answer all of the prompts, but you'll have to know what all of your hardware needs for devices and modules.

Probably google for:
xfree86 ati radeon 9700
to see what module to use.

Also maybe check out this page:
http://www.ati.com/support/faq/linux.html

HTH

~rock