Turning on AF assist will definately help.
Does your Nissin flash have AF assist too? If it does it'll be better than the little one on the camera.
Stofen for your Nissin :
http://www.amazon.co.uk/Stofen-STO-FEN-C/dp/B00009R69T/ref=sr_1_24?ie=UTF8&qid=1331513910&sr=8-24Yup, even a £2-300 lens will be better than the kit lens. Kit lenses (on this level of camera) are just a starting point, often they use cheap glass or even plastic for the actual elements, so going up one level in the quality of the glass will be a big improvement.
Also if you get a lens with a constant f2.8 aperture, you won't be forced to use higher f-stops when you zoom in, and in low-light shooting this means you won't have to pump up the ISO to compensate (which brings in noise).
If I get time, I might try a test shoot for comparison of my kit lens vs 50mm prime vs Sigma 28-70mm f2.8
On the subject of bounce flash, even if you don't have a nice ceiling, you can still use bounce flash, either just pointed up, or with a bounce reflector
like
this one or
this one.
Below is another (sorry) example of a shot taken with kit lens and flash pointing up, no bounce reflector, in a large dark marquee, but it still lights up the crowd. (Not a perfect shot, but you get the idea)
