Moving site to another domain name

I'm running a vBulletin forum with almost 100,000 posts and 3,500 members. The latest months there has been a tremendous increase in the number of new members, due to better Google visibility.

Now, I'm planning on moving the entire forum to a better domain name (long overdue). Will the negative impact on Google last for a long time before it comes back to the current levels of hits?

There will almost certainly be a hit while things get re-indexed. It may take an hour.... It may take a month.

Make sure you set up google webmaster tools to know the new address of your site.