For help configuring the default open-source ATI driver for use with Beryl or Compiz & AIGLX, see here (you only need to worry about the first page since Beryl is already installed in Cassandra):
http://www.howtoforge.com/ubuntu_feisty ... ati_radeon
The ATI proprietary driver only supports 8500 and newer series cards, so for your 7500 you will have to stick with the open-source driver. To use XGL you can install it from Synaptic and then you have to create a new log-in session for XGL.
I had the default open-source driver runing with Beryl on an X800 XT PE and it worked very well. The 7500 is considerably older and slower so it's hard to say how it would perform.