Do you have to uninstall old video card drivers? Not necessarily. It used to be given as advice that you had to uninstall them before you changed video cards, but that was also in the days of systems not always online, drivers only available on disks, etc. Nowadays, if you uninstall them and let the system do its reboot, they will get reinstalled again automatically when the system comes back up.
The way I'd do it is this:
1. Shut down the system
2. Swap the video card
3. Restart the system and follow the card manufacturer's instructions for installing the driver.
4. Look in Add/Remove programs and uninstall any software that went with the old video card.