I've thought a bit to how ZB operates IMO(no expert tho)
ZB partially substitutes open GL/DX with its own visualization engine, made through pixols. That's why its CPU /memory dependant but not so much graphic card dependant.
i see many apps video card dependants that have probs because of drivers , and new features added to them, but ZB has not that many problems in its case. only small quirks in 3.0 version. it doesn't generates normal or disp maps on the fly, i guess.
it just visualizes polygons thorugh the pixol environment so that it can be very rapid and powerful in sculpting response.
the app is also very optimized to work also oin common machines, and becomes really rapid when you have enough of ram and a good cpu.
could be wrong here, but apps like Modo , Silo and for now 3D coat are less powerful and smooth in sculpting for a similar reason.
but, the workflow is also what counts.
ZB is wonderful for creating characters and models from scratch with z spheres, sculpt and detail em to very fine levels, but its not the same in texturing/painting. no layers, depends on poly resolution, no classic uv unwrap and also doesn't accept overlapping uvs. its not that u can't texture with it, btw u can do wonders and has z app link for what's missing, but workflow is convoluted and just weird in this area for now.
3.1 retopo tools, as for now, work, but aren't on the same level of a Nex or of what 3d coat is going to have soon.