not very loud in my car?

Before you start messing with line amps and pre amps, make sure you understand how your iPod interface works. There may be multiple volume controls that affect how loud you the sound comes out of your speakers.

Without knowing what kind of "iPod interface" you have (and what kind of stereo you have), there's at least two volume controls that I know of: the volume control on your iPod, and the volume control on your stereo. Make sure to test the setup thoroughly by adjusting both of those volumes, but also look on the iPod interface itself to see if it has its own volume control.

Generally speaking, there shouldn't be a reason why you would need to put an amplifier in order to get loud sound from your iPod.

Of course, like Kyle commented, make sure to try a friend's iPod with the iPod interface, just to make sure it's not your iPod that's goofy.