I have the D3 and the D3x and shoot professionally with both of them.
The D3x focusses at the same speed as the D3, and up to 800 ISO is pretty much the same for noise, at 1600 ISO in reasonable quality of light (not amount - but quality) it is still very very usable. I'd say it was about 1-stop worse than the D3 over 800 ISO. FPS is fine at 12-bit, but I wouldn't shoot sports at 14-bit RAW because the FPS drops to about 1.5 per second - that is what the D3 (or D700 with grip) is for.
They are both exceptional cameras, the D3x produces exceptional files, which exhibit a different look than 12MP files even when printed at 10x8 or smaller it is hard to explain but the camera is a valued tool in my kit bag, and out on location not just in the studio.
But genuinely and seriously even with the ability to spend thousands more on a body for the use you describe I would suggest that the D700 + grip is more than enough for your needs as others have said.
I have a 21x14" print framed as a sample taken at 2000 ISO in near darkness with the 12MP D3, pin sharp full of detail and a 36x24" canvas also taken with the D3 also full of detail - albeit at 400 ISO in open shade.
Really you only *need* 24MP if you are looking for commercial or advertising work, regularly sell or print work at > 24", or need to be able to crop from a full frame and print large.
I'd save the money and spend on good glass, a flashgun to use when there isn't quality of light, and maybe some training or bank it and put it towards the next upgrade....
4 years ago you would have killed for the quality of image the D700 or D3x puts out at 1600 ISO, I think we all forget the limitations we had with film, and the early digital cameras.