hi there pajhonkawell first of all let me say this…zbrush is a software made by GOD!!!..aham now after thats been said i think you should first need to try it befor purchasing it to see if it suits your needs tho zbrush has so many various ways to be used that i think any one can find it to be an essential part if ones workflow.
now about zbrush and maya:
first you can create meshes or forms in maya and export your meshes as an .obj or ztl(not sure about this file type but im sure about the obj)file and then import or load them in your zbrush software. now im quite sure you can do the same with the zbrush files (tho once you got to the zbrush phase im not sure yull ever want to export it to another software for any purpose other then rendering
) but i havent gotten to this phase yet cuz un still working on my zbrush skills and havent gotten to the phase of rendering yet 
and for the last part of your question concerning the sys req:
in zbrush you have the abbility of sculpting specific areas in your model thus hiding the rest and making it faster and easier to sculpt that way without putting alot of stress on your system tho zbrush doesnt have any high sys req and i think your comp is quite good for it BUT i suggest download a trial or a demo just to be sure 