Do you have the Nvidia driver installed? (kmod-nvidia, akmod-nvidia or install from the nvidia site)
If so, you would need to remove the Nvidia driver.
To remove the Nvidia proprietary drivers (if installed using the kmod/akmod):
yum erase *\nvidia\*
If you don't have the Nvidia driver installed, and pretty much using the Fedora default, then it should detect the ATI card and load proper drivers for it when you boot.
However, you may wish to install the proprietary drivers once you get the ATI card installed.