vsmvsm,
You get GPS on your PDA because it is cheaper than getting a comparable standalone unit for your car.
If you are looking into this, then I suggest that you go with a bundle. The bundles typically come with the mapping software, a car mount, a car charger, and a receiver GPS.
I got my bundle for approximately $260 at buygpsnow.com. I'm sure you can get a cheaper setup because the price of the receivers has went down, and I think that the software is also down by $10. Expect to spend $200 to $300 depending on your needs.
Oh, a comparable system starts around $500, and it can get in the thousands.
If you are tired of getting lost or just like knowing exactly where you are going then I recommend that you look into getting GPS.
My recommendation is below:
http://www.buygpsnow.com/item.aspx?itemid=211
There is also a 10% discount.
Good luck,