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Get QuickTime Pro if you haven't already got it, you can use the quicktime player to record audio (and video), just make sure you set the prefs to a nice low codec, AAC 128Kbps should be fine for speech.

Be aware that the fans kicking in will be noisy however.
 
No, they'll kick in if it gets too hot, battery power will help as the PSU ramps up the heat.

QT is a core process, it's footprint is small, however Analogue to digital conversion is a little hungry.

Best way to check is to leave it running for a while, see what happens.

At 3 hours you'd better make sure you use a lossy codec.
 
Just get a little Sony digital recorder. They're like $80 and can record several hours of audio (up to 12 I think). I just hit the record button, record the whole lecture, press stop, go home, connect it to my MacBook Pro via USB, drag the .mp3 file into iTunes, and label it. No CPU usage at all. Pretty decent quality recordings, too.

This is the one I have.

http://www.sonystyle.com/webapp/wcs...10151&langId=-1&productId=8198552921665363982
 
Make sure you get permission to record your lecture. I've been in lectures where the lecturer has actually asked the culprit to leave. Lots of stuff that lecturers talk about is the subject of their latest book/article so they don't want people to record their intellectual property without permission. Most are fine if you ask them first. Just a friendly bit of advice. :)
 
The few times I've had to record a lecture, I've just used Garageband. It worked very well, as the mic is very sensitive.
 
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