>which one is more expensive, voxel or polygon or ellipsoids and how do we calculate/know the gpu expense

Existing videocards only work with poligons. At least used to, maybe some insane people made videocards specifically for voxels or something.

>how to calculate the RAM required to run a software from its codes and during programming

1) Run program
2) See how much memory it eats in resource manager or something