I just did a simple test to see how Nikon D90 does in low light. The following are links to 100% crops from the scene on the left.
The Camera: Nikon D90
The lens: Nikkor 70-300mm 4.5-5.6 VR
Lighting: Two fluoroscent bulbs behind the camera, low light, at night.
The objectives were two fold:
 See how worse it is to use in-camera JPEGs versus NEFs for low light
 See how well does the in-camera noise removal work.
As labeled, the first image in each row is from raw (NEF), second one is JPEG from the camera without any noise removal, and third one is with in-camera normal noise removal enabled. (You will have to click on the individual image sections below to goto to the full 100% crop images to see the differences. The blog editor downsizes them to fit in the limited space here).
ISO 400 (NEF, JPG, JPG-NoiseRemoval-Normal)
ISO 800 (NEF, JPG, JPG-NoiseRemoval-Normal)
ISO 1600 (NEF, JPG, JPG-NoiseRemoval-Normal)
ISO 3200 (NEF, JPG, JPG-NoiseRemoval-Normal)
ISO HI1.0 (NEF, JPG, JPG-NoiseRemoval-Normal)
If you would like to see some real life examples, please follow this link.