This completely depends on whether you shoot RAW vs. JPEG (noise reduction is automatically applied to JPEG images) and also what size you plan to use when viewing the images.
In a typical "web" size image, you often can't notice noise even if the noise is fairly strong. But at a "100% crop" (meaning it's enlarged enough so that you can see every pixel) you might notice fairly strong noise levels.
Here's a sample -- not shot with a 600D, this was shot with a 5D II, but it illustrates the point.
This image was taken at ISO 6400: http://www.flickr.com/photos/thevirtualtim/6304378772/in/set-72157628031936034/
As you view it, there's not much noticable noise -- the noise is there but it doesn't seem like much because of the size you're using when viewing this.
However... view the next image: http://www.flickr.com/photos/thevirtualtim/6304378966/in/set-72157628031936034/
This is not a different exposure... it's the same picture. I've just cropped in from the original so that you can only see his face, but this image is a 100% crop so you can see every pixel and from here you can notice the noise is actually fairly strong.
Note that these images were shot in RAW and I delibereately did not use any noise-reduction processing on these. Keep in mind that noise-reduction software also has the tendency to "soften" an image.
My guess is you will be quite pleased with any ISO of 800 or lower. At 1600 and 3200 you'll likely see some noise but it will likely be moderate and acceptable (and also fairly easily handled using noise-reduction software). At 6400 you will probably not like the amount of noise UNLESS you're just going to use small image sizes.
But keep this in mind... to keep noise low, you can keep the ISO sensitivety low and use a longer exposure time. But if the exposure time is too long for a hand-held shot then you'll get blur caused by camera motion during the exposure. While you can "de-noise" an image using software, you can't do much about a blurry shot because the camera was moving during the exposure. I'd rather have a sharp image with noise than a low-noise image that's blurry. There's a reason you can crank up that ISO and sometimes cranking it up is the right thing to do.
Lastly, keep in mind that a GREAT way to keep the ISO down and thus keep the noise down is to use a lens with a lower focal ratio so that it collects more light. If you were using an 18-55mm f/3.5-5.6 kit lens to take a shot at or near the 55mm end then the lens' widest aperture is f/5.6. If you were to use a 50mm f/1.4 lens, you'd have a lens that can collect SIXTEEN TIMES more light (f/1.4 is FOUR full stops more than f/5.6) -- but keep in mind that the depth of field gets much narrower so you may not want to go below f/2.0 depending on what you're shooting (although f/2 is still eight times more light than f/5.6). This is why they refer to lenses with low focal ratios as "fast" lenses - they can collect a lot of light in a short amount of time.
Tim Campbell
5D III, 5D IV, 60Da
