It was a few years ago since I shot at SxSW but I shoot regularly in Dallas in similar type venues and you will have a challenge in shooting these shows.
I would go with the fastest lenses that you have and you can forget about shooting at 5.6 pretty much unless you just luck out.
As to shooting at the venues forget about the Austin Music Hall and Antones-won't be able to get a camera into those places- may be able to get one into most of the other venues but be prepared to be stopped at the door at some and turned back, so just go to another that is camera friendly.
You should be able to use flash in some of the places so if you can get near the stage make use of the flash.
Just be prepaired for a lot of your shots to not work out for you, most likely the performer moving too fast on you. If you like Folk music they are the most condusive to nightclub shooting followed by blues, pop rock, some techno and a few others hardest to shoot are Metal Heads and Punk Rockers jumping all over the place!
Aren't wrist bands something like $200 this year?
http://2006.sxsw.com/music/showcases/