The upgrade to the D200 will not improve your low light photography - what you will need is faster glass.
So from your D80 and 50 1.8, I'd suggest the following:
Seeing as you're covering just the one event, you can consider hiring the 17-55 and 70-200 2.8. Both these will serve you well for the whole day.
The 10-20 Sigma is a good lens to own, and will give you interesting distorted ultra-wide angle group photo; it's an alternative to a fisheye.
I've a lengthy post on this thread here
regarding wedding photography. It's a challenge, and the right equipment helps. But I'd definitely make it clear that the upgrade from the D80 to the D200 is meaningless if you're only looking for better low light performance. That is not the way to go.