One concern about the flash, you might want to add a light modifier to your flash unit so your shots don't look like bunch of tapirs in headlights.
If the indoor ballroom's ceilings not too high, try looking for something like Gary Fong's thingamajig-tron Lightsphere http://cgi.ebay.com.au/Gary-Fong-Li...s?hash=item53dd71af84&_trksid=p3286.m63.l1177
and point your flash up. If it's too high, then just get a normal diffuser like this:
http://cgi.ebay.com.au/Diffuser-for...ries?hash=item35a3c3364c&_trksid=p3286.c0.m14
If the indoor ballroom's ceilings not too high, try looking for something like Gary Fong's thingamajig-tron Lightsphere http://cgi.ebay.com.au/Gary-Fong-Li...s?hash=item53dd71af84&_trksid=p3286.m63.l1177
and point your flash up. If it's too high, then just get a normal diffuser like this:
http://cgi.ebay.com.au/Diffuser-for...ries?hash=item35a3c3364c&_trksid=p3286.c0.m14