Consider contacting Virtual Ability in world. They recently helped create the online conference experience for MuseWeb, so they already have experience.
There's more to a successful online conference than just the virtual space, by the way. Creating the logistics so that attendees can mix and chat with one another outside of the scheduled conference presentations, making provisions for vendors - if you have been supported by them in the past, training presenters to be comfortable using a new technology, etc., etc.
I would be happy to share information and experience with you if you would like. dickd@innovaision.com