What is your OS? If you are in XP, you need to right click on the desktop--->Properties---->Settings. Then play with the resolution until you get it right. Some monitors give you the right resolution to play with. With Windows 7 you right click on the desktop---->Screen Resolution and then play with the settings to get it right. What monitor did you buy?
(Edit) You cannot upgrade graphics chips (Easily) in laptops as they are soldered in.