Upload & Sell: On
here is what thom hogan says on his blog about the question:
"So the first of the marketing messages is this: all else equal, FX probably gives you another stop of high ISO capability over DX. By that I mean that--assuming everything has been managed perfectly equal, which isn't always the case--when we make a 24" print from a DX body at ISO 3200 we should probably get visibly indistinguishable results from an FX body at ISO 6400. This also ties into one of my sub-themes lately: we have fast FX lenses, but we don't have fast DX lenses, so the differences tend to be actually higher in practice because we have FX folk shooting ISO 6400 at f/1.4 and DX folk shooting ISO 3200 at f/2.8 or higher. Still, the marketing message regarding the body is the same: FX is a better choice for very low light work, while the DX system should hold its own against FX in almost any other amount of lighting."
you can read more under the section titled "DX versus FX (again)" on his blog: http://www.bythom.com/index.htm