I am using ATI 3d Rage II PC2TV and had similar problems.
When trying to load Knoppix the screen went black. I noticed later it did the same thing when I tried Easy Linux and also failed on Win XP.
For some odd reason my problem was my monitor, even though it could go 1024 by 768 (to play Civ III).
I bought a refurbished Dell monitor and they all load fine now.

note: The old monitor still works with Mandrake,Win98,2000.....