hey there
from my limited experience, there's a couple things to keep in mind
1. is the subject still or moving?
2. do you have the luxury of using a tripod (in other words, are you still?)
Well, if the subject is still and you're still on a solid tripod (especially with a remote shutter release to minimize all movement--then film speed isn't that important. If everything is still, you might as well aim for less noise by using a relatively low ISO and keeping your shutter open longer to get the light you need. Of course, like someone pointed out, having a fast lens that lets in more light is nice too. The fast lenses tend to be prime or really expensive zooms.
this is frequently how night landscapes are done i suppose
OK, now, if you or the subject is moving and you don't want a blur, that's when the high film speeds are neccesary. Like, if you're shooting a band at a small club and don't have room for tripods, you might need that film speed 800 and higher to help compensate for some of the movement. Even with a 1600 ISO, in that situation it's often likely you'd still need to rely on a lower-than-desirable shutter speed. In that case, you just got try to be still as possible and shoot with a 5+ to 1 ratio in mind (take 5 or more shots of every shot you want) and hope some come out well. It's worked for me so far cuz I don't like the effect of lighting up a stage with flash and detroying the natural lighting effects.
anyway my two cents....