Re: Indoor Event Photography
spent sometime browsing at Canon USA...
i'd go with the F/4 IS...
the F/2.8 without IS seems like a choice, but I think IS would be important for a wedding..running, jumping around, etc...
Re: Indoor Event Photography
The important thing to remember about IS is that it doesn't actually increase your shutter speed, it just lessens the effect of camera shake at low shutter speeds. What this means is that if you're shooting action in a low-light situation, you're more likely to end up with blurry pictures (because of the subject(s) moving) with the f/4 as opposed to the f/2.8.
I've never used the 24-105 f/4 IS myself but I've heard lots of good things about it; I have the 24-70 f/2.8 and it's quite possibly my favorite lens.
I would say that a fast prime or primes (like the 35 f/1.4) are probably the most important thing you could get; if you had a slightly wide prime and a slightly telephoto prime (50 f/1.4 (on a crop body) or 85 f/1.8) you'd probably be pretty well covered; you could supplement those with the 24-105 if you still had any money left over. If you only wanted to do one lens, though, I'd say the 24-70 f/2.8.
As for flash, it's always good to have one (or more) around but you'll definitely want to do some reading about how to get good results with it. Sticking it in the camera hotshoe and pointing it straight forward is not going to give nice results. Strobist is a great resource for learning about the basics of lighting and how to get nice lighting setups with only flash units like the 580 (as opposed to way more expensive pro lighting rigs).