amplifier in built in usually around 5-8W. The sound coming out of you sound card on you computer, wether it be an onboard sound card or a external usb sound card would rarely go over 1W.
This is enough to hear the music if your locked in a sound proof garage with the engine off....Just...
So yes you will need an amplifier of some description.
If you have an external 5.1sound card, i highly suggest getting a 4channel or 5.1 channel amp
this will allow you from your computer to adjust the balance left and right and fade front and back on the fly.
Hope this helps
Luke ( OzCorp )