Fortunately, I could solve (very easily) my own problem: get that 1280x1024 resolution back ! All I did was:
sax2 -r (+ acceptation of the default config, without testing it or I would have received the usual black screen)
The new version of xorg.conf seemed to be easier to read...
Then, I modified all "Modes" lines in xorg.conf (Screen section) in order to get this:
Modes "1280x1024" "1024x768"
All I did was to add the resolution I wanted back ("1280x1024") in first position...
Ctrl+Alt+Del and happily, I could change the resolution with Ctrl+Alt++ and recover a decent resolution
I did not try to get the black screen again, in order to see if a lacking resolution in xorg.conf leads to this kind of crash.
For Adrian (if he didn't give up): I've read an important note in the howto you followed: "Unfortunately you need to recompile the "fglrx" kernel module right after any kernel (security) update. Use "fglrx-kernel-build.sh" for this".
Is the fglrx module loaded into your kernel ('lsmod' as root) ?
Is the 'Load "glx"' string present in the "Module" section of your xorg.conf ?