Today I shot an indoor play (children’s theatre) using the Nikon D3 and the wonderful (but heavy) Nikon AF-S 24-70 f/2.8G ED [specs]. Normally I shoot Zeiss ZF lenses, but in this case I needed the ability to rapidly change the field of view. Although I used f/2.8 for some shots, I needed f/4 and even f/8 for certain images. Get the Nikon 24-70 at B&H Photo.
Nikon D3 at ISO 6400 pushed 1 stop, click to see actual pixels
The lighting was consistent and even, but required ISO values 3200/6400/12800 and even 25600 for a few shots (even allowing the shutter speed to drop into the 1/40 sec range). The Nikon D3 can really handle jobs like this well. In the past, my choice would have been to shoot (other lenses) at f/1.4 and f/2 to obtain a decent shutter speed. At those apertures, depth of field is a challenge, focus errors become a problem, etc. The Nikon D3 makes shooting at high ISO values worthwhile. Get the Nikon D3 at B&H Photo. Or the Nikon D700.
There is noise, but it is not an objectionable noise, looking more like chunky film grain, although in really dark areas slight streaking can be observed. Click the image above to see examples. The 10" wide print I made had visible noise, but it was not at all objectionable.
One thing to keep in mind with high ISO shooting is that resolution is degraded by noise and also (in practice) by reduced depth of field, so there is little or nothing to be gained by shooting the 21MP Canon EOS 1Ds Mark III, which would have required ISO 3200, then “pushing ”images one or two or three stops in the RAW-file converter. The foregoing is at least a good theory, worth validating by research at some point.
Perhaps a Nikon D3f (“fast”) with only 6 fat-pixel megapixels would be a great camera for such situations; 6 megapixels can make very nice 24X16 prints for many types of subject matter.