I'd like to record the speaker and the slides separately (ideally just doing screen capture for the slides) and use an external microphone rather than rely on the laptop's internal one.

As for serving the talks after they've finished, I really like InfoQ's set up (see, eg, http://www.infoq.com/presentations/Simple-Made-Easy). Is there a system like this we can run internally since our talks often contain non-public information?