I successfully managed to make my X1550 work perfectly using the 300g Gallium driver, it involved installing from Xrog-edgers PPA, but I did all that in Ubuntu 10.10. Now, the 300g drivers come by default with Ubuntu 11.04, so that's strange!
First let's see what's being used, open a terminal and type:
sudo apt-get install mesa-utils
Then, run: glxinfo | grep -i opengl
Normally you should get:
OpenGL vendor string: X.Org R300 Project
OpenGL renderer string: Gallium 0.4 on ATI RVXXX
OpenGL version string: 2.1 Mesa 7.11-devel
OpenGL shading language version string: 1.20
OpenGL extensions:
Now let's make sure the drivers are in place, type these in the terminal:
sudo apt-get install xserver-xorg-video-ati xserver-xorg-video-radeon libgl1-mesa-dri