Almost all Laptops can't change video cards. In fact, there is no "card", but the video chips are soldered directed to the motherboard. Changing the video card means a different model motherboard. It can run from 500 to about 1,000 bucks to do this.
Be sure you buy the laptop with the video you want and not hope you can change it later.
Can I use ATI Rage Mobility 128-M 16MB in Dell Inspiron 5100 instead of 16 MB ATi Mobility Radeon 7500? If not, where can I find 16 MB ATi Mobility Radeon 7500?