I own the D3S and D700 with enough glass to get the shot right in-camera. I still have to crop from time to time.
I'm upgrading to the D4 for the improved focus and hopefully better performance above ISO 1600. The D3S is awesome; any improvement will help in Kuwait and after I get home. I've been shooting a lot of sports in low light before sunrise and after sunset. Most of my customers want email or screen saver sized output.
The D800 will have to wait until after I get home to Minnesota where the dust isn't so bad. 4 frames per second will be fine when I'm doing marco or landscape work. 36 megapixels will be nice for large prints. I normally print 13x19 so I'm not sure 36m will show.
Did I mention improved auto-focus? It is the same on D800 and D4. The D700 and D3S are the same too but don't operate the same for me. The D3S seems to operate much better.