680 sounds high enough (although not a very high score) to indicate that you already can run in accelerated mode and the problem might be with the game(s) you've tried.
Haven't tried running quake2 on my box but at least tuxracer works fine.
(I haven't got the exact same card but mine uses the radeon driver too)
One game included in the Knoppix distro you could test with is Chromium.
I think it only uses 2d acceleration though but it really sucks without it.
Also, commenting Option "sw_cursor" and uncommenting Option "hw_cursor"
( in /etc/X11/XF86Config-4) might inprove performance a bit.
(it might also make X unusable, if so turn it back the way it was from a console )