Reading between the lines, you seem to be very happy with your D90. So why upgrade?
Since you only list the 18-105mm kit lens in your User Profile, which is a DX lens: You will have to invest in FX lenses to take full advantage of the D800 or any other FX body. I would recommend investing in fast glass before you consider a D800 body. Keep in mind that glass is a long term (lifetime) investment that will hold its value long into the future. Camera bodies are upgraded about every two to three years and their value drops like a rock. Once you have a decent collection of lenses, upgrading to an FX body should be a lot less painful (financially). Good Luck and Enjoy your Nikons!