Agreed on the 50mm I picked mine up brand new for 119. Its a lens every photographer should have
For low light, the 50mm is excellent!!
Depends, what sort of photograph you want to shoot.
Do you want to shoot indoor sports? Or do you want to shoot events (like dance performances, gigs, etc) etc etc..
It also depends on where you sit. If you are in the front row the 50mm would be perfect (just as an example).. But lets face it how often do you get front row seats. So a nice zoom lens will come in handy. Pic one that has a large aperture)
Also comes down to your budget.
But for low light shots, you would want the aperture to be large (small f-number, eg f1.4-2.8).
Edit: How good is that tamron 17-50 with close up shots (macro)? The sigma 17-70 is sorta like a macro lens (well i guess it is).. Will focus on objects pretty close up!!