Segui estrita/ os passos dessa dica p/ o Ubuntu Gusty Gibbon, encontrada no fórum internacional do Ubuntu:

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Well, let's try here and see if I can help you... If yes, I will create a Wiki page.

Before we begin -- supported hardware list

Taken from fglrx driver description for Gutsy.

This version of the ATI driver officially supports:

* FireGL: V7350, V7300, V7200, V7100, V5200, V5100, V5000, V3400,
V3300, V3200, V3100, X3-256, X3, X2-256, Z1-128, T2-128,
X1-128, X1-256p
* FireMV: 2200 (Single card PCI-e configuration)
* Mobility FireGL: V5000, T2
* Mobility Radeon: X1800, X1600, X1400, X1300, X800, X700, X600, X300,
9800, 9600, 9550, 9500
* Radeon Xpress: 200M series, 1250 IGP, 200 series
* Radeon: X1900, X1800, X1600, X1300, X850, X800, X700, X600, X550,
X300, 9800, 9700, 9600, 9550, 9500

ATI All-in-Wonder variants of the above cards/chips are also supported,
but video capture is not.

If you are doing fresh install of Gutsy

   1. Enable fgrlx driver.

      Install linux-restricted-modules and restricted-manager provied in the restricted repositories:
      Code:

      sudo apt-get update
      sudo apt-get install linux-restricted-modules-generic restricted-manager

      Open the restricted drivers manager in "System -> Administration -> Restricted Drivers Manager" and select "ATI accelerated graphics driver".
   2. Install xserver-xgl package
      Code:

      sudo apt-get install xserver-xgl

   3. Install compiz
      Code:

      sudo apt-get install compizconfig-settings-manager compiz compiz-core compiz-fusion-plugins-main compiz-fusion-plugins-extra compiz-gnome compiz-plugins libcompizconfig-backend-gconf libcompizconfig0

   4. Reboot
   5. Log in. 3D effects should be enabled!
   6. Customize Compiz Fusion.
      Select System → Preferences → Advanced Desktop Effects Settings
      In the new window, General Options → Desktop Size → Horizontal Virtual Size. Set it to 4.
      The other two options have to be left at 1.

      Continue customization per Forlong's guide at http://forlong.blogage.de/article/20...-Compiz-Fusion

Fonte: http://ubuntuforums.org/showthread.php?t=569654&highlight=x300+gutsy

Final/ a minha ATI Radeon x300 PCIE passou a ter o 3d e também o famoso Compiz Fusion.
A única coisa que está pegando é a tela do login que está vindo em 1920x1080, e eu uso um monitor LCD 17'' c/ a resolução 11280x1024 c/ taxa de resolução de 60 Hz