will you focus on CPU or GPU rendering? I think the latter is a better bet for now (giving more choice for other render engines as well) What kind of work do you do?
The new 30 series cards from Nvidia seem to be very promising in terms of price/performance. For Rhino look for best single core performance. Ram: depends on what else you want to do at the same time as running Rhino, how many instances of Rhino you want to have open, how large are the files you work with, both in size as wel as in amount of objects in your Rhino scene.