I take quite a few photos during Arctic winter when the sun is away, so I have to use high ISO, up to 1250, but I prefer to stay at 640 with M9. The important is to ETTR. I use LR4.x and normally apply some Lum and Col noise reduction (<20). I turn off Detail and Sharpening, but increas Contrast (in reduction). In my limited understanding this leads to an evil spiral. Increase in sharpening leads to more noise leading to...How much noise reduction to use depends on print or WEB export. As a rule of thumb I turn up reduction until satisfied in LR, and then reduce it a bit before exporting, and even more for prints.
Dont know if this helps, please also help me with additional / correcting info :-)