If you're experiencing hangs while running with desktop effects (i.e. compiz) enabled on a system with an ATI video card using the OSS radeon driver, you may want to try out the workaround suggested in the very active bug on launchpad regarding this issue: downgrade your ATI driver from 6.7 (Gutsy) to 6.6 (Feisty). This worked on my ThinkPad T42 system and allowed me to use compiz unfettered.
Ubuntu's ATI driver packages are available here:
http://archive.ubuntu.com/ubuntu/pool/main/x/xserver-xorg-video-ati/
Here are the 6.6 i386 and amd64 packages:
http://archive.ubuntu.com/ubuntu/pool/main/x/xserver-xorg-video-ati/xser...
http://archive.ubuntu.com/ubuntu/pool/main/x/xserver-xorg-video-ati/xser...
To install the i386 package, issue something like the following in a terminal:
$ wget http://archive.ubuntu.com/ubuntu/pool/main/x/xserver-xorg-video-ati/xser...
$ sudo dpkg -i xserver-xorg-video-ati_6.6.3-2ubuntu6_i386.deb
$ echo xserver-xorg-video-ati hold | sudo dpkg --set-selections
The dpkg --set-selections invocation prevents the driver from getting upgraded to the broken version again the next time you update packages on your system.