For me, it all depends on what lighting the band will use on the night.
I know people who play in very good semi-pro bands and they either have their own lights or they get in a bloke with his own gear. The standard can be very high and the best known lighting bloke round here uses multiple lasers, smoke machines and goodness knows what else.
If this is the case then I think it shouldn't be too difficult to get decent shots without any flash or anything like that. When I shot a very good semi-pro band last year, they had their own lighting and I just set the ISO to 1600 (the maximum for my camera), set the mode to shutter priority with a speed of 1/60th and the results were quite reasonable. I expect you'll have better kit than me so under these conditions you'll get good shots.
If the venue doesn't have that kind of lighting then you might struggle. In all the time I've been watching bands in pubs etc I've never seen anyone use a flash and I think bands aren't that keen on them. I would ask the band first before you use a flash and make sure they're O.K. with it.
Good Luck!