The answer is easy. The D800! It will make you a 50% better photographer than a D3x. Ask any of the people who place advanced orders.
However, if you're more based on reality than fantasy, then the answer may be different. The D3x has many advantages over the D800, body build, as someone said earlier, being foremost among them. I've dropped my D3x twice, from 3-4 feet, onto asphalt and rock. Both times it emerged unscathed (though scared the crap out of me). If the D800 is anything like the D7000, I am sure the outcome would have been different.
Also, the D3x can use all three PC-E lenses without limitations. The D800 should be able to use the 45mm and 85mm, but will have somewhat limited use with the 24mm.
My biggest complaint with the D800 is the external batter pack/grip. Not matter how well made it will be, it will never be as solid as an integral batter pack like D3x. And, as it will be what sits on the tripod, that means there is an increased potential for camera movement. I've seen this on the D7000.
The bottom line is that the D3x is made for professional work. It is tough and and it is reliable. The D800 is made for advanced amateurs, thus is made less robust. However, a new D800 is probably less expensive than a used D3x and money is almost always an issue.
As others have said, I see no reason to trade in my D3x for a D800.