I noticed that on the Audio How-To page at http://wiki.gumstix.org/index.php?title=Category:How_to_-_audio, it says that an amplifier is needed for Expansion Boards without a stereo headseat jack in order for a microphone to be added.  

Could anyone point me in the right direction as to which amplifier I should buy?  I am using a Palo35 Expansion Board.