what is the best way to start modelling a realistic car?

hi i want to try and make a mustang GT or Audi r8 for a upcoming contest and was wondering where is the best place to start when modelling a car by doing it all in one, or in separate parts?, and how would i go about texturing it for the max possible realistic look? also what is the better external renderer? yafaray or luxrender? or is there anymore tht i could use for the most realistic render?

A car modelling tutorial series http://www.blendercookie.com/2009/12/05/modeling-a-porsche-911-gt3-rs-part-01/
For the renderer, why not blender internal, like this one http://www.youtube.com/watch?v=-Yp7II3XjW0&feature=player_embedded
http://blog.mikepan.com/the-making-of/

thank you for the tutorial they will come in handy
also was tht done with blender internal? tht looks so good i didnt think blender internal was good enough for realistic renders like tht?