I use an "old" GeForce Ti 4200 and do not make complex render, but many times I see the GPU quality equivalent or even better than the CPU (FireFly).

I see GPU quality getting better and better and I believe eventually it will get there (100% photo realistic quality).

Regard the support, I undestand Microsoft implemented an interesting layered concept in DirectX:

The render software (FireFly) "talks" with DirectX that "talks" with the GPU.
And DirectX acts like a translator also. If the GPU has the ability, DirectX use it, if not, it is rendered by DirectX (including OpenGL).

Sure, quality will vary, but getting closer to 100% (photo-realism) there should be not that much difference for most.

But the only 2 GPU vendors (nVidia and ATI) are aligned with Microsoft, so GPU and DirectX are evolving in sync.

Just talking about image quality (when talking about speed ... there is no comparation).

Marcio