If you are going to amplify the output of an iPod to big speakers, as others have said, you need an amplifier. Which means you want the cleanest output possible, so don't take it from the 3.5mm headphone jack, but from the dock connector. You can buy a $50 apple version or a generic $7.50 version like this:
One advantage of the apple unit is it includes power so your iPod does not run out of juice.