When I take low-light photos with my Sony NEX-5R, I find that I stick to ISO 200 to ensure acceptable noise.
I was told that fear of ISO will hold me back, and that I should do noise-reduction in software. I always thought of noise-reduction as reducing detail (there's no free lunch), but maybe I'm wrong, so I wanted to check: how much does noise-reduction let me increase ISO without a noticeable loss of quality -- either an increase in noise, or a reduction in detail, or some other aspect? If I shoot at ISO 200 without doing NR in post, can I shoot at ISO 400 if I'm doing NR in post? 800? 1600? 6400? I expect to not notice a difference if I switch back and forth between the two photos.
I have Lightroom 5, and Nik Dfine, but not Topaz DeNoise (though I'm willing to try it out and buy it if it makes a huge -- rather than incremental -- difference). I don't have Photoshop. I'm also willing to use other software, provided it's not command-line, runs on the Mac, and is cheap (like $20, not like $100).
Is there a rule of thumb (like: "NR lets you increase ISO 8x"), or is the answer just to try it out?
I don't print my photos, and I don't view them at 100%, either, but full-screen on my monitors.
Here's an example of the kind of photo for which I noticed the noise:
At ISO 400 or above, the sky (not visible in this crop) is no longer black but grayish / noisy.