no, do not format the Drive, rather go and look for ALL your ATi drivers and remove them, then try reboot, after that, make sure your nVidia drivers are working and tryout a game to see if it is ok.. if not, your GPU might be overheating somehow, but i doubt that would happen.
ive never needed to format and reinstall windows after putting a new gfx in.. so i dont know why some think you must do it...