well, no, you won't have to reinstall you're OS.
yes, you will have to uninstall the drivers for your old graphics card.
the general procedure when changing a graphics card (provided they are not both from the same company) is:
1. With the old card still in your computer, uninstall the existing drivers.
2. turn your computer off, replace the old card with the new one, make sure everything is plugged in (extra molex connectors etc.) then boot up your computer.
3. Install the drivers that came on the cd that was bundled with your graphics card - or use more up to date drivers that have been downloaded off the appropriate site.
hope that explains it