I want to have another go at 3D within XNA. What I have seen from some other games that they just have a separate very low poly model "cage" around the environment model. However I can not find any reference to this. I have not that much experience with XNA 3D either.

Is it possible to have this cage within each of my environmental models already? Lets just say I call the mesh within the .FBX wall and col_wall. How would I call to these different meshes within XNA?

The player would just have a tight collision cube around. To make it a bit more efficient I will be making divide the map up by cubes and only calculate collision if the player is in it.

Question two: I can't find anywhere to do cube vs mesh collision. Is there a method for this? Or perhaps it is possible to build my collision cage out of cubes in the 3D app and on loading of the models in XNA replace them directly by cubes? So I could just do box to box collision which should be very cheap and still give the player the ability to move over ledges on the static models.