Most rendering engines are written to use the CPU for calculations. You would need to find a rendering engine specifically written to use the GPU for rendering. Search "Nvidia Gelato" for an example. It takes a lot of practice to get good using any particular rendering software, so I don't know if its worth your time to learn a GPU-written rendering engine. But if you're making this a hobby, then go ahead!
What 'softmodding' a Geforce card will do is to make your computer system think it has a much more expensive, Nvidia Quadro card. This is done because some programs (like 3DS Max) have better viewport performance using specially designed drivers for these cards. Search "Maxtreme" (but again, viewport performance isn't the same thing as rendering a final image)
Hope this helps.