How high can I take ISO settings for large prints with a Nikon D3S?

I wanted to know a bit more about this, as the Nikon brochure for the camera has amazing prints at ISO 12800 at around 11X14 or something like that.

What could someone expect if they needed to print something like 24"X36" or bigger? Would you stop at ISO 5000 for that size, providing you could reduce noise properly?

You would want to keep the noise to a minimum on larger prints, and the D3S is very good at doing that.

Just how high an ISO you would use would really depend on the scene you are taking and how well it is lit… If the camera's amp is having to work hard to show detail in the image then it's likely to be more noisy than if there's some form of lighting - Signal/noise ratio. In general it's considered best to keep the ISO low to reduce noise artefacts.

The D3s is amazing when it comes to ISO performance. If you want to enlarge really big keep the ISO down as much as possible. I've had great results at 3200 ISO. USe some noise reduction in LIghtroom or get a program like Noise Ninja to reduce noise a little.