According to my dogpile search, 128 DDR ATI radeon 9800 pro is AGP. I take it you have a Dell computer? You can go to Dell's web page and confirm by searching support on your particular model number. Replacing the video card is easy. Take the old one out, put the new one in. You should clean up the drivers using Driver Cleaner (free) from http://www.drivercleaner.net/
If I were going to replace the video card, this would be the sequence of events. Make sure the current driver is available to install. Go to add/remove programs and remove the video driver. Then, boot into safe mode, run driver cleaner TWICE, each time deleting all that it finds. Then shut down and remove the old card, install the new one. Then reboot and install the new hardware and driver.
Just curious, how did you reach the conclusion that you need a new graphics card? You could try doing the driver clean up detailed above on the card you already have. It might solve your problem.