How to Upgrade to 10.6.8 without breaking the 5770/5870 video drivers
Unfortunately the 10.6.8 update completely disables some 5770 video cards, and disables the DisplayPort on other 5770 cards and 5870 cards.
If you have already upgraded to 10.6.8 and have lost display functionality, please see this article.
There is a patch that fixes the broken drivers in 10.6.8. The trick is to install the patch after installing 10.6.8 but before restarting the computer to apply the update. If you restart and complete the 10.6.8 before installing the patch, you may find yourself with no display. Read on for details.
Warning: Follow this procedure at your own risk! The smart thing to do is to fully back up your 10.6.7 boot volume before proceeding.
1. Instead of updating to 10.6.8 via Software Update, download the 10.6.8 installer from Apple here. (This installer assumes you have 10.6.7 already installed. If you have 10.6.6 installed, you can download the 10.6.8 combo from Apple here.)
2. Run the installer, but when the installer finishes, DO NOT CLICK RESTART. Before restarting your machine, run the 5xxxDriverPatch. After installing the patch, you can safely restart.