I would wait until Z4 is out for a bit before considering upgrades.
What is said above is pretty much true, ZB doesn’t make much use of the graphics card. However, if you’re going to take your models to something like Maya, Max, Modo, C4d, etc., the graphics card is pretty important tool.
Overall, we use Nvidia graphics cards very heavily these days. Except on a few stations, we don’t use the workstation high-end models as much anymore because the price/performance is somewhat marginal. I would look at Cinebench outputs for something like the new SLI models such the Geforce 285/295 models, which seem to give a lot of bang for the buck. (I, myself, still work mostly on two computers that are equipped with the Geforce 9xxx series, and don’t find myself paticularly hampered by the graphics adapater.)
-K