It seems that actually the problem was with fglrx driver installation that was done by me in those days.
Even if I disabled it in xorg and pc startup scripts, the fglrx kernel module loaded itself at boot and probably caused problems with the radeon driver used at X startup....
Solved deinstalling at this moment the proprietary driver.
Gianluca
It seems that starting at 23/04 with xorg-x11-server-1.4.99.901-23.20080415.fc9 I'm not able to use EXA accel anymore and so no xvideo with my Sapphire X1900 card.