There are a few things to consider before choosing the D800, I have one and I have also had a D3 and a couple of D3S - the D800 is an ideal landscape/studio camera.
Pros:
It has a very high resolution so can provide high quality images.
Because of the large file size it is possible to crop considerably and still end up with a very good image.
The large file size allows a very large printing capability which can be useful when selling printed images, canvases etc.
Its low-light ability (ISO) is very good.
It is
currently pretty latest technology.
Cons:
It demands good glass, it will show up any flaws in lenses much more so than other cameras.
Linked to the above any shortfall in technique will be exaggerated and can result in less sharp images.
The large files will make further demands on your PC and file storage, and the size or number of cards you use.
It has a low frame rate (though it can be increased slightly with a grip and crop mode).
The D3/D3S/D3X range is larger in the hand and some people like the feel and grip of the larger body - full 'Pro-spec' so a rugged option and with battery life second to none.
Of the three the D3S has the greatest low-light ability and is a superb camera with a high frame rate, both the D3 and D3S have 12MP resolution, which is in fact still brilliant for anything except perhaps
very large prints - many Pros still use a D3S in preference to anything Nikon has yet tried to replace it with.
The D3X at 24MP and was again primarily designed as a studio camera so doesn't have anything like the same low-light ability of the D3S and has a lower frame rate.
I use a D4 and a D800 but if I was forced to chose between my D800 and a D3S I would take the D3S.
Much will depend on your available/intended
quality lenses, your technique and most importantly what you intend to use it for.
Hope that helps