If you would like to have the best and less expensive car kit use these:

Tape adapter (same as used for iPod)
Headphone adapter (the one used to hear music in Treo)

I placed these in order to hear music in mi car and it works perfect, but when I was listen my music I received a call and for my surprise it works like a handsfree and you are able to talk perfect and you hear through your car.
At least it workes perfect with my Treo 600 I am not sure if it will work with 650.

And imagine the cost of it!