You don't HAVE to uninstall the AMD drivers before you install the NVidia Card in your socket, but it is a good idea to avoid errors occuring if the AMD drivers try to do anything with the NVidia card!
I suggest that you download the latest NVidia drivers from their website (search for your specific card on the driver download section), leave these in your downloads folder ... DO NOT install them yet. Then uninstall the AMD drivers (the computer will run in 800x600 framebuffer mode after this). Then swap the cards to the NVidia one, reboot and install the NVidia drivers you downloaded.
Its generally a good idea to always get the latest drivers for all hardware from the manufacturer's website!!
The boost clock isn't necessarily always going to be the same, like in your example +50 mhz. It will be higher or lower depending on the amount of power headroom available to the card. It really depends on the game being played and the temperature levels of the GPU.