NVidia has even a tool which shows you the result simultaneously, get it here: http://developer.nvidia.com/object/melody_home.html NVidia had another normal mapping tool, but it seems they donīt provide it any longer
Yes, the PS Normalmap Plugin has a preview version, where you can load the original texture and see how it will look "ingame". And NV Melody is only for making a normalmap with a high poly model and not with a texture, not? oO