C++ - design question

Answer 1

Use a base Geometry class:

class Geometry {
public:
    virtual ~Geometry() { }
    virtual void Render() = 0;
};

and have each of your Geometry-type classes derive from this base class and implement their specific rendering functionality by overriding Render.

Then, Renderer::RenderGeometry does not need to be a function template; it can simply take a pointer to the base Geometry class and call the virtual function Render.

Answer 2

Template is not neccessity. If you think hard about it, most of the time templates only do text-replacing and is a safer macros.

OOP was not design to rely heavily on templates, but composition and inheritance (like what James suggested)