For Rhino, you have to look for good single-core performance, since it doesn’t really take much advantage of multicore CPUs for now. This doesn’t mean that you can’t get a multicore processor, but a high clock rate (i.e. … GHz) is favourable over a high number of cores!
This is fine! Usually the more memory the better, but Rhino - in my experience - isn’t that memory hungry. Other programs are though (i.e. everything Adobe)!
The GPU would probably be worth to upgrade to 8 GB (if possible) of memory, especially if you want a fast raytraced view or quick rendering with Cycles! Many render engines support GPU rendering nowadays to take a load of the CPU. I guess that’s something to keep in mind, if it concerns you. If not or you only very rarely render anything, then your current choice is fine, too.
The Rhino app takes about 1GB of hard-drive space, the rest really depends on your personal storage needs.
I use both platforms and although the Mac version of Rhino is close to, and has a few enhancements over the windows version in some areas, it really depends on what you do in Rhino if going for a Mac is a good choice.
