I have an ATI XFX Radeon 4670 1GB video card in a Windows 7 Ultimate x64 setup. If I only use one monitor, all is well. If I plug in a second DVI cable to the card, both monitors go dark. I can get one or the other to display, but only when it is the only one plugged into the card. I'm really hoping for a dual-monitor setup here as that was the point of the card. I'm using ATI Radeon HD 4600 Series video drivers dated 11/24/09 (8.681.0.0) which are the latest from their site and are supposed to work with Vista or Windows 7 x64.
When the computer boots with both displays connected, both monitors display the inital BIOS and boot screens. The Windows 7 splash screen also displays, but by the time it goes to the login screen, one monitor is offline.