My understanding, from article mentioned below, is that high ISO settings actually produce more deterioration of image quality in low light conditions than bright ones. As in low light conditions the benefits of ISO50 becomes stronger than ever, you are encouraged to stick with that and try to find some means of dealing with the slow shutter speed that will result.
If you go to :
click on the red "Articles" on the right then on the drop down menu click on "High ISO Settings".
If you're a software freak too KW2003
you might want to look at the free noise reduction software at http://www.neatimage.com