There are several known problems or limitations related to the XFree86 ATI driver. They include:
Improper mouse protocol specification with some recent mice. Try different protocol specifications or another mouse.
A system conflict with APM. This problem is Linux-specific. There is a bug in kernels 2.0.31 or earlier that prevents proper APM operation. Upgrade to a more recent kernel or disable APM support.
The TV port on some Mach64 adapters needs to be disabled using an ATI utility that might or might not be supplied with the adapter. This problem is currently under investigation.