Go to System->Administration->Hardware Drivers and check the box to enable the restricted drivers for your NVIDIA card if the option is provided.
If the restricted driver remains unactivated after attempting to activate it in the Hardware Drivers dialog, you may not have the appropriate linux headers installed to compile the driver. Ensure that the linux-headers-XXX and linux-restricted-modules-XXX packages are installed, where XXX matches the version of the kernel you are using (linux-image-XXX).
If the activation hangs on download/install dialog, you can install the driver using System->Administration->Synaptic Package Manager, make sure you pick the latest driver version recommended by the "Hardware Drivers" tool and all its dependencies. Go to Hardware Drivers tool and activate the driver you just installed.
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