Open a console and type

<code>
gtf 1280 1024 70
</code>

(replace the "70" with your desired frame rate)

That should give you a line like
Modeline "1280x1024_70.00" 108.88 1280 1360 ...1024 ...

Your values will, of course, vary. Copy that line into the Section "Monitor" of your
/etc/X11/{XF86Config-4|xorg.conf} (I don't know which one is used in Knoppix-4).
Then change the lines in that file in section "Screen" that read
Modes "1024x768" " ...
to include the "1280x1024_70.00" at the beginning. (Look for an entry that says
DefaultColorDepth <number> in the screen section and edit at least the corresponding
Modes in SubSection Display for that depth.

The next start of X should result in your desired resolution. If not, read carefully through
/var/log/{XFee*|xorg*}.log for error messages.