I have read some tips in ubuntuforum.org, but I encountered many unexception.
It's being said that ATI(fglrx) Official Driver was not fully compatible with some XGL extension, especially the cards higher than 9500(R300).
XGL suport Nvidia 5XXX and 6XXX series OK!, but it's not full finished. So you can wait some time and keep eyes on it. ATI promised will suport XGL in future.