Flash is a must for dark party situations, unless you have a body that can cope with ISO 1600 + with minimal noise and a lens that goes wider than f2.8.
Here's an example, taking in the same setting, of the same person actually.
With flash and kit lens: ISO 400, 1/60th, f5.6 @40mm
Untitled by
AlistairBeavis, on Flickr
Without flash, 50mm prime lens: ISO 3200, 1/25th, f2.2
Absorbed by
AlistairBeavis, on Flickr
As you can see, the one without flash is soft and has an orangy hue to it (down to the ambient light in the restaurant). If you want to capture expressions and movement in such a setting, you really need a flash.
Seriously, you should spend a little money so you can do justice to the event. I certainly wouldn't want to do a wedding with just a kit lens and a 50mm.
But you don't have to spend a lot.
My flash is a Sigma EF530 DG ST and cost me £40 from ebay.
The shots above were taken before I got my 18-50mm f2.8, hence I was stuck with the kit lens.
My Sigma 18-50mm f2.8 only cost me £100 (s/h obviously) but it's a great lens for the money.