There are only a few cameras on the market with better high ISO performance than the D7000 - they are the D700, D3, D3s, and D4. The D800 will add to that list and pick up a stop on the D3x and a half stop on the D7000 (due to downsizing for comparative purposes). So in the scheme of things, the D800 is very good with low light.
The D7000 is perfectly fine for most situations at ISO 1600. Even my D300 has been used for published images at ISO 1600 if the size is kept moderate. But for fine art prints or cropped images, there is noise even at ISO 800.
The related issue is dynamic range - which is better at lower ISO levels. In an ideal world, I'm trying to shoot at ISO 400 or lower with a D7000. That translates to ISO 1600 with a D700 and ISO 640 more or less for the D800.
Hopefully Bill Claff can get his hands on some files for comparison.