I have seen BIMx used or mentioned, but what is the actual difference between BIMx and BIM?
You mean xBIM -go to www.openbim.org
eXtensible Building Information Modelling - although my personal interpretation for BIM would be Building Information Management.
There is a lot of BUM around too :) (Bloody Ugly Modelling)
This format is only applicable to ArchiCAD, which allows you to create 3D models in an .bimx format. The file can be viewed with viewer applications (on Mac OS X, Apple iOS and Windows)