Some animation programs will use the PhysX SDK, so having an Nvidia card can definitely show benefits if you decide to use it. I'm sure you must have a GPU in your PC already, and if it's Nvidia you probably don't need to add or change anything. If you have ATI you can run a dedicated PhysX card if you want, in which case anything around 100 CUDA cores is just about perfect (GT 240, 9800GT, GT 440) and you'll also need the GenL V1.04ff patch (Google it).
That said, there's other ways to do physics stuff. Bullet does gpu accelleration AFAIK, and doesn't require a certain brand of card.