You will find lots of evidence in publications and online reviews (and the answers here) of the following:
** Nikon's high-end range of cameras have technically superior sensor technology for high-ISO shooting.**
I am not sure if your eyes will tell the difference at ISO 800.
Pretty sure that nobody could be sure about whether a shot was taken on a Canon or a Nikon at ISO 400 or below, with their eyes.
Shooting at high ISO above 400 is the last resort for exposure adjustment when the goal is clear, crisp photography. So you have to consider whether you will need to use ISOs higher than 400 often to get the shots that you want (are you shooting action in low light, or long telephoto lenses 200mm and longer?). Otherwise, the cure is longer shutter speed, wider aperture, or a tripod/image stabilization.
Nikon provides better value dollar for dollar in their camera body technology.
Canon has a much more extensive range of camera lenses and add-ons (both from Canon and from third parties).
I would only consider these top end cameras (D3, D3x, 5D Mark II, and 1Ds Mark III) if I were being provided equipment by an employer for a gig... perhaps to go shoot wildlife or to do studio photography.
If it were wildlife, I would go with the Nikon D3(x) because the high ISO will benefit shooting distant or moving objects in uncontrolled lighting situations.
If it were studio shooting with controlled lighting, or product shots, I would put the cameras back on par because high ISO does not matter in this environment.
If I were talking walk-about photos and candids in uncontrolled lighting situations, I would get a D90 with an 18-200 VR, or an XSi with a 24-105mm IS.
If I were shooting weddings, i would use those same cameras as backups to either a D300 or a 50D with a prime lens around 100mm.
I hope that you get the information you need to make your decision.