I think you should go for a 70-200 f/2.8 or if to pricey the f/4.0 will do too. Wide Lens can be cool too, if you're able to get near the artists, if not you can save on that. I'd prefer not to use a flash, if its to dark you have to go up to ISO800-ISO1600, at least the pictures should be okay (don't know how the 50D performs on such levels) if you're not going above ISO2000.

Depends on how close you can get but the 70-200 will work well here I think.

I think you should go for a 70-200 f/2.8 or if too pricey the f/4.0 will do too. Wide Lens can be cool too, if you're able to get near the artists, if not you can save on that. I'd prefer not to use a flash, if its to dark you have to go up to ISO800-ISO1600, at least the pictures should be okay (don't know how the 50D performs on such levels) if you're not going above ISO2000.

Depends on how close you can get but the 70-200 will work well here I think.