First, find out the wattage of your power supply ! Then get a good ATI Radeon card that will work with your power supply and fit . You could just upgrade your drivers and more. Go to Nvidia's site and find your card and see what you can do to get peak performance. Save some money that way !
Well some laptops you can't actually change the graphics card unless you really dig into it, i recommend you research if the graphics card can firstly be changed, secondly you must find the exact size of the graphics card and get the relevant graphics card you wish to upgrade to in that size, but I'm sure you cannot actually changed the graphics card because its normally built into the motherboard.