As it depends entirely on the NB in question. Some have replaceable Video Cards and others have the Video Chip Set included on the M'Board. Depending on the NB that you have it may be possible to change the Video Card but generally speaking it is not possible to add another one.
You really need to look at the Service Manual for your NB here and see what is possible.
Though if you have a Nvidia Video in your NB it may be covered sometime shortly by a Recall. Currently Nvidia is having problems with some of their GPU's which appear to have a design problem with them. If these are recalled it will involve fitting a replacement M'Board to most NB's currently in service.
If you have a HP or Compaq NB you should look on the HP Web Site for Updated Video Drivers. While these drivers will not improve the actual Video Display they do switch on the fan at a lower Temp to prevent overheating issues with the Nvidia Video Chip Sets.
Col