I primarily agree with Neuroanatomist, in general terms.
What I would suggest is to look at the photos you already have and look at the general focal lengths you are using. You also said the ISO performance wasn't satisfactory. What f stop would bring your ISO into a better range? If you need to go wider than 50 mm frequently and f2.8 will be fast enough, then the Tamron 17-50 is likely the best bet because of the versatile range. If f2.8 is not enough, you will need a prime and need to have a solid understanding of what range or ranges. This, of course, assumes either you will be shooting these low light shots handheld and/or you are shooting people (who have the audacity to move) in these low light shots (it sounds like that is the case). Otherwise, slow glass isn't a big deal with a tripod.
The 50 mm are wonderful, but you mentioned tight indoor shoots so that may not work as well as you hope.