Are you using 32-bit or 64-bit? Is it compatible with your hardware?

I ran into problems getting 32-bit Ubuntu to recognize more that 4 GPUs which lead to booting problems, but that was after the install.

 

If you are still having problems..when the machine starts to boot into grub, press tab as it starts in and edit the entry to boot linux...remove quiet and nosplash and then add nomodset. Ubuntu sometimes tries to load the ati driver when it is not properly installed for some reason. Doing this will allow you to get into the console and install the right packages before X starts and hangs again.

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Orignal From: Are you using 32-bit or 64-bit? Is it compatible with your hardware?