Well, when you're saying poly modelling, do you mean the method where you start from a plane and extending the edges to have a form?
If so, this tutorial might help you:
http://67.15.36.49/team/Tutorials/fiat500/fiat500.asp
and I'd mention that the end results of boxmodelling a car and polymodelling the car should roughly be the same. What is really important to consider is how you manage your polygons on your model such as their spacings, regularity, etc.
Good luck!