I don't usually use auto ISO but I would say 1600 would be max for me. Shutter speed could be bit slower for you, maybe 1/60 although be careful with 55-200 especially in the long end. Is it the VR version or regular ?
"Sunset is always an experience - but check the ISO speed!"
I almost always use Auto-ISO. I think it is the best invention since sliced bread, but I also understand why others feel differently. I don't have the D5100, but I do have a D3200 which has a similar sensor as far as high ISO quality is concerned. I use a maximum ISO sensitivity of 3200 most of the time and feel very comfortable about using 6400 in certain circumstances. I also set my minimum shutter speed to 1/125.
That being said, I can echo what the others have said, qualifying their answers about needing higher shutter speeds for action shots and longer focal length lenses. I also concur with what Marty has said about turning off Auto-ISO, or lowering the maximum setting, when you are using flash since it will almost always choose the highest setting in low light conditions. You might also want to read through a couple of the recent posts about flash and Auto-ISO settings.
Marty is also right about needing an almost perfect exposure at the higher ISO settings.
First I will qualify my answer since I don't have a D5100. I don't use Auto ISO either. I always use the lowest ISO setting possible for the given subject and ambient conditions. The best settings (both shutter speed and ISO) is dependent on your subject, the focal length you are using, and the ambient conditions. 1/125th sec. is a reasonable minimum shutter speed for general photography but too slow for shooting sports or shooting at focal lengths longer than 100mm or so without VR.
The D5100 should have no problem producing clean images at ISO 1600 or even a little higher if you nail the exposure.
Auto ISO and Flash do not play well together. When you shoot using flash, you should always turn off Auto ISO and set the ISO manually. Good Luck and Enjoy your Nikons!
thanks Janne,JP,Marty,CoolMom42, Both the Lens 18-55mm and 55-200mm are VR. For sports and higher focal length, I change the shutter speed to 1/500. That being said, I have set the ISO Sensitivity Settings as follows :
ISO Sensitivity - 100 (base) Auto ISO Sensitivity - ON Max Sensitivity - 800