ATI has released a hotfix for its Catalyst 8.12 drivers, this update fixes a problem with two or more Radeon HD 4850 graphics cards in Windows Vista. You can download it over here.
After installing ATI Catalyst driver release 8.12 for two or more ATI Radeon HD ™ 4850 graphics cards in a system with ATI CrossFireX™ technology, the system restarts and halts on a black screen. The black screen occurs prior to the loading of the desktop for the Windows Vista 64-bit or Windows Vista 32-bit operating systems.
This issue ONLY occurs when two or more ATI Radeon HD 4850 graphic cards are installed in the system with ATI CrossFireX technology. One single ATI Radeon HD 4850 card will not encounter this issue.