As far as I understand ZBrush is still a pure software renderer. Which means your rendering time as well modeling time will not be decreassed by adding a faster graphics card. Your amount of RAM, and the speed of that ram, + CPU power will determine how much you can get out of ZBrush.
ZBrush is a software renderer so CPU power should come foremost. But if you continously need to increase your polygon count beyond your current status quo then invest in some more ram. It is a balance act between CPU and RAM, not the graphics card.
Remember you can always hide some of your geometry to speed up the realtime deformation of the remaining model.
Graphics cards definetely come in second place when talking about ZBrush.
There are some ZBrush functions that require a modern graphics card. If you can already run ZMapper then do not worry about it.