Below are 100% detail shots showing the noise in infrared shots taken with the Nikon D200 at varying ISO settings. Here I used the green channel as being most indicative of the noise:
As you can see, noise is very well controlled through 800ISO. In all cases I had noise correction turned on, which significantly extending the shooting time at the lower ISO settings as a dark exposure is made following the actual exposure.
Below are some more images taken with the D200:
As you can see, the D200 is capable of some lovely IR photography, though it is not suitable for portrait and wedding IR work without either modification or with a very fast lens and high ISO setting.