Like XP, Vista let's you use multiple monitors on a system that has multiple video cards installed on it. Unlike XP however, Vista has a new feature called Windows Aero that displays windows using a translucent "glass" style, displays taskbar thumbnails, and has Flip 3 and other cool display stuff. But can you get the Aero experience if you have two or more monitors/video cards on a Vista computer?
Yes, provided all your video cards use the same WDDM video driver. Windows Display Driver Model is a new driver model in Vista that supports Aero effects, so to get Aero you need a video card that has a WDDM driver. And to get Aero on both monitors in a dual monitor configuration, both of your video cards have to use the same WDDM driver. This means for example that you can't have one ATI card and one nVidia card in your machine as they come from different manufacturers and therefore have different drivers. And if both of your cards are from ATI, then they have to be either identical cards or similar enough to be able to use the same driver in order to get Aero working on both of your monitors.