Which lens to buy for events

I currently own a Canon Rebel Xti with a Canon 17-85 IS 4/5.6 lens. I’ve been doing portraits for around 6 months with backdrop and several AB monolights. The lens produces very nice results and my customers are quite happy with their photos. Now I want to branch out and try some event photography like small weddings and various other parties. I’m thinking of replacing my lens with a new Canon 17-55 IS 2.8 lens for the indoor events. (can't afford both right now) I would have an advantage in low light conditions but lose the extra focal length that I’ve become so accustomed to having. I'm also looking at the Canon 24-105 4.0 IS lens.
I would welcome any suggestions. I want to stay with Canon lenses though.