Im currently shooting with a D700. I went from D80 to D300 and now the D700 for about 6 months.
My reason was also for the low noise in high ISO’s. My experience with it so far has been great.
I have have mostly been doing street photography the last couple of years and have been shooting at f2.8. But a couple months back i got a nice little tip from another D700 user.
I take street portraits of old / homeless people. So i like to pick up every little detail. So i change my ISO to 800 or more and my aperture to f8 or higher while keeping a fast shutter speed. This way i get loads of detail – BUT… i still get a clean, noise free file because of the D700’s great performance at high ISO.
You mentioned that the MP is a concern. Its not a huge upgrade. Even going from the D300 to D700 is a nice little upgrade just based on the censor sizes.
I read in some article that the more pixels you squeeze on a sensor will affect the quality. So its 12mp on cropped censor or 12mp on a full frame sensor. So i chose the full frame sensor ;-)
Another thing to keep in mind is the lenses. When putting on a DX lens on the D700 body. The D700 goes into DX mode. So it crops into the center part of the image.. losing pixels… i think it goes down to 5MP.
So you will have to buy FX lenses.. Its not such a big deal.. Your 50mm will work great.
If your worried about pixels… Go for the 5D MK2.. Its a bit cheaper and its 20something MP..