Depending on your machine, you'll have a maximum number of polygons/vertices available per subtool.
As for the blockiness, it's going to boil down to the topology (wireframe) of the base mesh on the lowest subdivision level; 6 million points might not be enough to sufficiently produce the detail if the majority of those points are being spent inefficiently (which might happen if you tried to sculpt a full face from a sphere, for example). In this case you could use Remesh/Dynamesh to try and get a more unified distribution for the topology, or you could manually retopologize the face (externally with a program like maya/topogun, or internally with zspheres) in order to work in more efficient edgeloops.
A common workflow might be to roughly sculpt out the major/overall form and features of a head, retopologize it, and then continue sculpting in the details. I think this is what is done in Eat3d's intro to zbrush 3.5 dvd.