Best lense for "Low Light" shots?..

I'm using a Canon Rebel XT.
I'm an amateur. I never use the flash!.. I'm using the Canon 50mm 1.4 lense.
I love taking shots indoor or outdoor using the "existing light",
and with my XT, I usually use ISO 1600.

Yes, there is some noise, but I have no real objection to that
because as long as I've "captured the moment", I'm okay with the
noise. And the the Rebel XT, the noise is not even that bad.

My question is: With the 50mm 1.4, I've got to get really close to the
performer in order to get a good shot.

But, for the times when I'm sitting in the rear of the theater/club, I'd love to have
a zoom lense that would permit me to shoot in all this "low light"...

I don't want to pay a thousand dollars for this mysterious lense,
so I'd appreciate any info on this subject.

Thanks.....