@gster;
If i would make an 4096 *4096 snapshot would that give the detail i need or must i go even higher.
And will 300 dpi be enough?

I export them as tiff's(or targa) yes, but i'm not a hero when it comes to (advanced) mental ray so i don't know what you mean by converting those to .map files nor what to do with them.

edit; I just got Z-Brush3 and somehow the displacement maps are not as defined as in zbrush2 and i will look for the update

Thanks