|The DRI RPM mini-HOWTO|
Before restarting X, make sure you have the right lines in your /etc/X11/XF86Config-4 file. The key ones are:
Section "Module" [ ... ] Load "glx" Load "dri" EndSection Section "DRI" Mode 0666 EndSection
By way of an example, this is my XF86Config-4 file.
Run startx and follow the above procedure to determine whether DRI is running. If it isn't, shut down X and check which modules are currently loaded, using lsmod. Try removing the appropriate module (in my case, r128) using rmmod r128. If this doesn't do the trick you will probably need to reboot before restarting X.