This happens EVERY time I install Ubuntu. I'll pop the live CD in, and install it. The graphics work fine until I have to install mandatory updates and then restart. From there, I'll be unable to even log in, as the login screen is a glitchy mess, save for a row of pixels at the top of the screen that are actually clear.
The only way I can circumvent this is by installing a bunch of pre-release updates and switching to a proprietary driver...however Steam games, many GUI elements, and screenshots won't work, or they'll be immensely laggy and glitchy.
What can I do to ensure optimal performance and stability for an ATI Radeon HD 6850 (1GB) graphics card?