Why are movies for example toy story, high poly and not video games? And How do I make it high poly like that in blender without running my computer to hard?
your first question seems to have two parts that are unrelated. Toy Story is high poly for rendering purposes, those same high poly models were not made for real-time (game engine) purposes.
You can use the Multires for added geometry to your model for adding detail by sculpting. Multires will allow you to work with high poly models on lower “levels”.
http://wiki.blender.org/index.php/Doc:2.4/Manual/Modeling/Meshes/Editing/Multires
Maybe you mean something like using the Subsurf Modifier and creating extra loops cuts for detail? i.e. take a simple model and make it smooth and add extra fine detail with cuts. 