> FX will be a big improvement over DX for... > DOF
Like Brian says, different DOF characteristics. Better? If you want generally less DOF, then yes. If you want more, then not so much
>wide angle shooting
Here I think the difference is somewhat mitigated by the large range of excellent DX ultra-wides. Indeed, considering that the widest-of-the-wide is the Sigma 12-24 FX, they make a DX-equivalent 8-16 and this levels the field. Probably the only thing you can get in FX is 14mm f/2.8 where there really isn't anything that wide and that fast for DX. Probably the FX advantage is really speed in ultrawide, which is not so much an advantage if your ultra-wide shooting is stopped down.
>noise at high iso
True, but be careful because if you have to crop to DX you negate this advantage. This has the corollary that if you're focal length limited on DX, then you need a longer lens on FX to gain the high-ISO advantage. This may be a key point...
>I am currently using my D90 in ISO 1600 and a 70-300mm VC 4.0-5.6. >What is likely to get me more improvement at the long end: 1) using a >D600 in ISO 3200 with my 70-300 4.0-5.6 VC or, 2) using a D7000 in >ISO800 with a 70-200 2.8. I guess the cost of a D600 is more or less >the same of D7000 + a Sigma/Tamron 70-200 2.8
At what focal length and aperture are you currently shooting your D90 & 70-300VC?
If you're at the back of the auditorium shooting at 300mm, then you need a 450mm lens to take advantage of the full FX sensor. You can crop your D600 image, but cropping to the same FOV as DX will pretty much negate any IQ improvement. This becomes a wash.
D7000 with 70-200 f/2.8? Probably a better plan, but again if you're moving from 300mm to 200mm, with 1.5x less focal length, you're losing a little more than 1 stop in the deeper crop you'll need to make. But with a lens that's 2 stops faster, at least you are still gaining 1 stop.
You could also shoot 70-200 on your D90 and really not be that much worse off. Also, unless you're talking about the Sigma OS version of this lens, you'll lose VC which means you may need to stabilize the lens somehow.
Ultimately, it's still difficult for FX to top DX in cases were you're shooting the longest practical lens in DX already. And telephoto in low light, you're into the realm of very expensive solutions. Getting closer and shooting with an 85/1.8 on your D90 may be the best possible solution. (not getting into Nikon 1, which combined with 85/1.8 gives you a 230mm f/1.8 FX equiv)