lens - What DX Nikon lenses should I look at for concert/club photography?

I'm looking at upgrading my kit for a music photojournalism internship over the summer and I was wondering what the best way to spend $600 was.

My current setup is a d7000, a kit lens, a manual focus 1.4/50mm and a speedlight. I'm looking at the 1.4/50mm AF, but I figure my money could be better spent, and I've gotten used to manual focus: http://minus.com/mW9z07wnQ#1

I'll be shooting small, dark 1,500-person venues -- so a 2.8/80-200mm is out of the question.

Ideas? Maybe get a diffuser for my speedlight? A fast 35mm? I've been looking at that Nikon prime fish-eye, but I'm skeptical about its use in photojournalism... Ken Rockwell recommends the Hemi plugin -- any thoughts?

Thanks for the help!