Go here: http://www.guru3d.com/category/driversweeper/
1) Download the Driver Sweeper program and read the directions./ Download the newest Nvidia driver for your card and save it to the desktop.
2) Uninstall the ATI driver when you are ready to install the new card.
3) Power down the PC and unplug it.
4) Swap cards.
4.5) Install PSU
5) Reboot in safe mood. ( Usually F8 at boot )
6) Follow the directions on the Guru3D website/run Driver Sweeper
7) Reboot into Windows and install the new Nvidia driver.
8) Enjoy new card