I have put Ubuntu on my mum's desktop computer. Unfortunately, the open source radeon driver does not work well with the onboard ATI graphics, and ATI's proprietary driver no longer supports the hardware at all. In order to use the ATI proprietary driver with this hardware, it is necessary to use an older version of Xorg, which is now only available in versions of Ubuntu older than 8.10. Unfortunately, the open source radeon driver seems to be causing X to lock up intermittently when my mum uses Audacity.
I'm willing to accept that some hardware is not well-supported on Ubuntu, and so, because this is a desktop computer with a couple of free PCI slots, I think a better solution might simply be to plug in a new graphics card that might have better driver support, and to disable the onboard ATI card in the BIOS. The requirements for this card are that it be inexpensive and have robust (preferably open source) driver support in Ubuntu 10.04. Heavy-duty graphics processing power is not a requirement. A second-hand card on Ebay would also be fine. Can anyone make some recommendations?
I've used Nvidia cards in almost all my Linux computers. The earliest one was an Nvidia TNT2 PCI. I've also used a GeForce4 MX 440 PCI. I had no problems with either of them. Here is a link to the older Nvidia cards that their 3 Legacy drivers support:
http://www.nvidia.com/object/IO_32667.html
Once you install Ubuntu, you should get a notification that an Nvidia proprietary driver is available. You would then just choose the legacy driver that matches the card. After the driver is installed, reboot for the computer for the new driver to be used.