Just need an individual amp for each speaker, or set of speakers if that is what you are after.
What frequency range & volume level are you after?
Speakers like thishttp://www.mpja.com/prodinfo.asp?number=14618+SP
put out a nice sound playing Arduino Tones, am sure would be good musically also.
Tho if you want lower frequency something bigger would give better base.
These little amps could drive 2 columns each, just make sure to wire up your speakers in appropriate series/parallel combinations (probably want to end up with 8 ohms, but doesn't appear to be listed).http://www.parts-express.com/pe/showdetl.cfm?Partnumber=310-300
Or browse around, they have other smaller amps also.
Or, find a surround system, they have 5-6 outputs, get 2 of those and might be less expensive.