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.