This is a Vista memory reporting issue. The NVIDIA Control Panel properly reports the dedicated video memory as 320MB. Please check with Microsoft for updates that may fix this in the future. For more information about this, please visit the link below.
Incorrect memory reporting on Geforce 8800GTS 320MB in Vista
Updated 04/12/2012 02:38 PM
Geforce 8800 GTS 320MB: Driver reports incorrect video memory (256MB) in Windows Vista Display Properties panel.