It's difficult to answer without an explanation of what it is your animating...
A complex scene, with dynamics/fluids/hair, or character animation with a complex rig will slow any machine down...
The simple answer would be to buy the fastest CPU you can afford - but your performance is ultimately going to be limited by the complexity of your scene. So learn to work efficiently, using place holders and proxy objects for character animation, and low-rez sims (for dynamics) until your at the rendering stage.