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This is what I'd do for the iPhone 3G (or original, for that matter): Get the $10 Belkin headphone adapter with mic/clicker for the original iPhone. It works the same for the 3G. It plugs into your iPhone's headphone port, and the mic/clicker works just like the one on your included iPhone earbuds. Then, get ANY car stereo at all that has a front auxiliary input, and plug that into the Belkin mic/clicker. This will allow you to:

Answer calls with the clicker
Talk without a headset/holding the phone
Hear your conversations and music seamlessly through the speakers.

Then, if you also want to charge it, you can get whatever cigarette lighter charger you like.
 
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