Z-brush can handle objects with 10-12 million polygons with ease on fairly “home grade” hardware… you can get several sub objects each with that sort of count with little problem…
The usual approach with this is to re-topologise the model and then bake normal and/or displacement maps… you can do that inside z-brush, in blender or using 3rd party tools like xnormal…
this is how anyone would do this in any package if doing a movie for example… in blender a combo of the subdividion surface modifier and displacement modifier will do the trick…
another possibility (blender specific) is to import the highpoly model into 2.49, then have a low poly retopologised version on another layer…
on the retopo one, add multires and subdivide up to a high poly count. then use the shrinkwrap modifier with the sculpted layer as the target… play with teh params until you get a good fit and then apply the modifier…
you will now have a multires version of your z-brush sculpt in blender… not much use as in 2.49 you can’t animate a multires objects with an armature for example…
Aha, but open the file in 2.5 and the multires gets turned into a modifier and you’re good to go! set a viewport level, a sculpt level and a render level!
This would all be smoother if you could do all the steps in 2.5 but sadly you can’t apply the shrinkwrap modifier now that multires is a modifier…(that can only be done in 2.49)
Don’t disregard blender though, the recent changes to sculpt mean I can sculpt a 6 million poly model on my crappy laptop with ease and good response speed…
the clay tools in z-brush are a thing of beauty! the one in blender is a bit broken really, but usable with a bit of luck and fiddling…
With any luck someone will work on the feel of the sculpting tools in blender, but Z-brush is worth the money as it has real breadth and depth in it’s tools for sculpting, layering, transferring data, resymmetrising, baking etc etc etc… if only it worked on linux!