I run a dual head setup, 2560x1024 Xinerama'ed. I was running the Ubuntu-stock deb 2:8.543-0ubuntu4 on plain Ubuntu 8.10 X86_64
Previously X would fork :0 and heap space would eat up 205MB per process. I suspect that there was some userspace FB going on.
Pulled the latest Cats from AMD's website and I'm now at a mere 65MB with ONE Xorg thread

I'm a happy camper now.