The setup for Webcasting with an iMac is incredibly simple.  I only use the following:-

• Logitech Desktop USB Mic (AK5430)
• Nicecast 

Screenshots of the settings I use to broadcast
Audio Setup in Nicecast

Nicecast does a very nice job of archiving (but it doesn't hurt to have a backup recording either - you can use Audacity)
Archive Setup

Once the broadcast is finished, I open the Nicecast Archive file and edit it, add music etc, using Audacity. 

The iMac does have an built in microphone but I find that the Logitech Desktop USB Mic gives me a little bit more control over background sounds.  It can also be muted very easily by pressing the control button on its base.  Very handy at times!