For precision modeling, the shortest advice is : define yourself the scale you'll be using for your model. For example, I've always used "1 Blender unit stands for 1 meter". So any precision like, e.g., a 5 mm gap between 2 body parts on a car is reachable : it's just, for that example, a matter of 0.005 Blender unit.
For a longer advice, one top level reading is the book from Robert Burke. He was published in BlenderArt some issues ago, and the book is a longer version of his works. There was a new on Blendernation about it :
Precision Modelling PDF Guide at BlenderNation. The guide is available on his site :
rab3D Home
About the native pixel resolution of Blender, it's (as far as I know) only used for background texture, and the resolution is 512 pixels for 2 blender units (with a zoom factor of 1.0) Such background textures may be used for blueprints, however I always used a different way of handling them : please see my website about it, as I've wrote a tutorial about it,
Pixels or Atoms ?.
Why ? Because I don't like to resize the blueprint with Gimp (or any other software) and I also want to use, as a modeling canvas, the actual dimensions of the car I'm working on. Length / width / height of course, but also wheelbase, tracks and tires dimensions like width, rim radius, sidewall height.