The Rage 128 card doesn't have a fixed amount of memory assigned to either 2D or 3D operations. The 16 MB of memory is assigned dynamically, as needed for either operation, meaning that any memory not used for your 2D operations will be made available for 3D operations.
Note: This information doesn't apply to other ATI-based chipsets, such as the Rage II, Rage IIc, or Rage Pro used in some other Apple products.