You might already be knowing that final rendering and/or post rendering is entirely a CPU intensive job.. However, scene rendering smoothness depends on the GPU and also memory.. The more complex the scene, the more memory you require.. As for the GPU part, you should be looking at the workstation class of video cards with atleast 512MB video memory on board (preferably 1GB depending upon the scene).. The ATI FirePro V4800 is an excellent buy in this class.. If you are going to indulge in animating hair, fur, clothing etc., physics would play an important role.. Nvidia cards with CUDA (if you can code for it) and physics would be of lot help in this case.. However, a decent Nvidia workstation card will cost way too much..
I say go for NVidia Quadro as it works great and provides you CUDA which will work, in some cases, to help speed up rendering and/or post rendering. You will have to do a bit more research on this however to see if it is compatible with your software.