Hey a subject I can speak on
I am currently on complete system switch #10 or so - it's fun, but does cost money in the end. Oh well, I am not rich but I usually buy used and sell at FM so things aren't too expensive and I get to see the capabilities of lots of systems..I grow tired of it now though and probably am stopping at my A77 setup.
The perfect system - It does not exist! My perfect system would be as follows - Canon's glass choices, Nikon's bodies, with Sony/Olympus/Pentax features in the body.
For the OP - what are you not liking about ISO6400 7D files? I have never owned the camera, only a short rental, so I've never done enormous comparisons.
However I owned the D700 three different times - very good camera, high ISO is pretty darn good but even at ISO 6400 things start turning a bit bleh, though not nearly as bad as crop cameras.
However, I have to wonder if you would downsample the 7D after some mild NR, would it help the files?
Are you printing or pixel peeping the shots? Just curious.
Also, I would be very hesitant to buy Sigma OS 70-200 when a Nikon VR 1 can be had used for about the same price (yes I would take a VR1 used over the Sigma OS new).
Consider the D700 maybe a stop, stop and a half above the 7D or so, this is before downsizing. Is it worth the hassle for you?
I think only you can answer this question - I love the D700, awesome great camera overall but it is pretty low pixel density, which may or may not matter depending on your output.
I don't like Nikon skin rendering, but an X-rite can help things a lot in this area.
Also, D700 has fairly strong AA filter so you have to sharpen a bit more than some other cameras (no big deal, but something to be aware of).
Good luck!