Thanks for the useful comments. I gave it a couple of tries using a bare-bones xorg.conf with and withoud explicitly enabling composite, but again no luck. I attach the bug reports for both configurations. If I'm reading the docs correctly, Centos 5.4 must be using xorg 1.1.1, which I guess is a pretty old version.
Could this problem, mentioned in another thread, be another manifestation of the same bug (since RHEL 5 uses practically the same binary packages)?