Can anyone tell me the difference b/w Packed Geo (created by Pack SOP) and Packed Fragments (created by Assemble SOP). Is it just that one has been tagged with a Name attribute?

It mostly comes down to the differences of how the two are represented in memory. Other than that, there are some limitations that apply packed fragments but not packed geometries. The way the alembic exporter treats them is slightly different as well.

http://www.sidefx.com/docs/houdini/model/packed [www.sidefx.com]

FYI: the pack sop can create geometries and fragments. The assemble sop is just an HDA that uses the pack sop within. I don't really use the assemble sop though, as it is not needed if you maintain naming/partitioning yourself.