I'm using an ati-x1300 with the fglrx driver setup (AIGLX etc.).
I notice that I get some black/red flicker when I try to run GL apps like Tremuluous or Crack-Attack.
I think the CPU may also be working quite heavily when this happens.
Running a Sempron, 2 GB RAM.
EDIT - it isn't black/red flicker, that just seemed to be an artifact of my background wallpaper. It occurs when I try to play video clips in VLC also.