Ok, I did a fresh install of Ubuntu 10.04 LTS. Well, "mostly" a fresh install since I only formatted the root partition and kept my /home partition. So I booted the new Ubuntu using my old profile.
So I'm still using my old desktop prefs.
On first boot, everything is fine, I still have my Compiz Cube, only the SeaMonkey icon is broken BUT my eyes hurt... the (CRT) screen is at 60 Hz refresh rate!
I check the available resolutions, all other resolution (including bigger ones) offer several refresh rates except for 1280x960 which is locked to 60Hz... No luck, that's my favorite.
Ok, so I go to /etc/X11, looking for xorg.conf file. There's none.
Ok, then I copy my backed up xorg.conf containing my custom '1280x960_80.00' modeline to /etc/X11/.
I restart X and... no more flickering, the screen is clocked at 80Hz. But... visual effects are disabled... no more desktop cube!
I try to re-enable them, it doesn't work.
Worse, OpenGL apps seg fault and Compiz doesn't start.
I check for libGL.so... There're two of them, one in /usr/mesa and the other in /usr/fglrx...
Since my ATI is a Radeon X1600 XT (RV530 chipset), it's not supported by ATI anymore so I'm using the GLX driver, not the FGLRX...
I specify it in xorg.conf and tell him to use ati driver (I also tried radeon and radeonhd). No change!
So I launch Synaptic and remove all fglrx packages (fglrx, xorg-driver-fglrx, fglrx-modaliases), and set xserver-xorg-video-radeon to "reinstall".
After that, I restart the computer and guess what? Everything works wonderfully!!! (I'm still using my custom xorg.conf to avoid the 60Hz problem but it only features monitor and screen sections)
I got flicker-less picture and visual effects enabled with 3D acceleration.
So if you have Compiz problems with an old ATI Radeon video card, uninstall the fglrx packages!
By the way, you can also do it command-line style:
sudo apt-get purge fglrx xorg-driver-fglrx fglrx-modaliases
sudo apt-get install --reinstall xserver-xorg-video-radeon