dear sir/madam:
i'm delighted that you have accepted our invitation to speak at the conference in [city] on [date].
as we agreed, you'll be speaking on the topic... from [time] to [time]. there will be an additional minutes for questions.
would you please tell me what kind of audio-visual equipment you'll need. if you could let me know your specific requirements by [date], i'll have plenty of time to make sure that the hotel provides you with what you need.
thank you again for agreeing to speak. i look forward to hearing from you.
sincerely yours,