Formal Event shoot

I've been asked to shoot a formal military ball coming up in a few weeks and I was wondering if there were any tips for getting good shots with even lighting in what will probably be a very dim lit environment? I intend to get either a 580EX or the Sigma 500DG Super for the event, but do I need to get a omnibounce or should I try to get one of those softbox type diffusers? Do I try to bounce the flash or can I shoot directly and what options are available on the flash for diffueing or toning down the light?

Any help/tips would be greatly appreciated.

You will need to scope out the space that the ball will be held in, but I suspect that the facility will have high ceilings and walls that won't be close to you. That makes the Omnibounce (and all other 'tupperware' style diffusers) totally useless. All they will really do is reduce the light hitting the subject if there are no nearby reflective surfaces (ideally white).

I suggest that you look at a LumiQuest Promax System kit. It gives you several choices which you can make on site based on conditions. Here's a link to their web page: http://www.lumiquest.c​om/lq931.htm