-For single monitor and mobile systems
64MB of graphics memory to support up to and between 1280x1024*
128MB of graphics memory to support up to and between 1920x1200
256MB of graphics memory to support over 1920x1200 (512MB or above recommended for 2560x1600 displays)
-For single card dual monitors
128MB of graphics memory to support up and between 1280x1024 x 2
256MB of graphics memory or more to support larger dual monitor configurations.
* Or equivalent in terms of total pixels. http://windows.microsoft.com/en-us/windows7/What-is-the-Aero-desktop-experience