Nvidia s a very solid dedicated graphics card provider. If a card fits, be sure to make sure there are quality drivers for your OS and make sure that there will be no significant battery or power issues. Otherwise, nVidia is a good choice.
Right now the Nvidia 8800 GT is blowing its competition out of the water. It is fast and stable but for a price of about 200 US dollars and up. It all boils down to what you are using your computer for. If its intense gaming Nvidia is your best bet. If not ATI also makes a decent product.
Yes you should. Nvidia has excellent driver support and better performance than most laptop cards from ati. You should check the compaq website to make sure the graphics card is compatible as there are different standards such as mini pci and mini pci express. ati driver support is improving but nvidia is still the best.
Most of the time Video Cards in Laptops (I use to own a Compaq Presario 2200) are built right onto the motherboard. You can go out and buy a new Video Card, but you may not be able to use it if the default Video card is APART of the motherboard of the laptop. Check this first!