According to Nvidia (http://nvidia.com/view.asp?IO=game_giantsopt), the following enhancements were made in the Geforce 3 version:

" - Significant performance improvements: tuned for the GeForce3 and DirectX8.0
 - Added higher resolution textures
 - Added increased polygon-count characters
 - Vertex / Pixel shaders
 - Meccs are now bumpy AND shiny
 - Moved terrain bump map calculations from CPU to GPU
 - Added soft edges to character shadow
 - Added sky reflection on water
 - Smoothed transition from water edge to terrain
 - Added weather effects!!"

There is a patch that upgrades the Geforce 2 version of Giants to the Geforce 3 version, found in the patches section.

GeForce 2	GeForce 3