No, installing a video card properly should not void your warranty. It is just a matter of getting into your BIOS, usually by hitting the "Delete" key while the computer is booting up, then navigating to where you can change the video from being on-board to the PCI (AGP) slot. You can navigate around in the BIOS with the "arrow keys" on your keyboard and use the "enter" key to open each section It is not hard to do and just requires you to think about what you are doing. Once you have changed the settings you will exit out of everything, by usually hitting the "Esc" key and the computer will ask you if you want to "Save your settings". You will type in a "Y" (meaning YES)and then exit the BIOS. You may have to hold the ON button IN to turn the computer off as it may try booting back into windows.
Now you will take the side cover screws and cover off and to where you can install the video card you bought into the computer. Heres a tutorial on installing a video card.http://www.directron...howtovideo.html
Please read it carefully to understand what you are going to do. Ask any questions you want here as no question is a dumb one. Its better to ask beforehand so you will understand what you are doing better when you try to do something.
After you get your video card installed you will button everything back up and restart the computer. You will get a very ugly 16 color video and will then use the cdrom included with the video card to install the video drivers on your computer, then restart the computer again and you will be set with the new video card.