It entirely depends on your type of photography. The d3s is a small step up from your d3 as explained earlier. The d3x would be a better camera for the landscape stuff, or any scenes that have a lot of detail you want to bring out. The d3 is a good high iso camera and fast frame count. The d3x is an excellant low iso camera with a much lower frame count. I would think the d3x is a beautiful camera to have for especially shooting slow from a tripod for structural or landscape type stuff.
If it were me, I would keep the d3 and buy the d3x. If you have been using professional cameras with the built in grip then the d800 would not be my choice. I tried the d800, it wasn't the camera for me. I had the d3s so I upgraded to the d4 and haven't looked back.