My iMac is about three years old. It's kicking along fine, as long as I don't do anything too hardcore in the 3d department. We're having slight issues there, as in Blender starts lagging and acting strangely (like failing to realise that I've "released" when the pen is off the tablet) when it has to deal with over 2M polys.
This is the machine I want to get:
Mac Pro