Ok, I have not tested this yet, as soon as I did I will update this posts, so please if you try it and have good results (or bad) post it on comments.
First get the ATI drivers from its page, here:
follow the instructions given there for CentOS
Then you will need the Fedora Extras repository enabled in your yum configuration.
So create this file:
with this content.
[extras] name=Fedora Core 6 Extras mirrorlist=http://mirrors.fedoraproject.org/mirrorlist?repo=extras-6&arch=$basearch enabled=1 gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-extras gpgcheck=1
Remember to turn this repo off once Beryl is installed, I think is better that way, so you do not mix packages when updating your CentOS
Then save it and run.
yum install beryl-gnome
That should be all, please remember to comment here if you get any good or bad results.