I guess until Ge-force 6 you could do a full MOD to a quadro. With ge force 8 you still can do a soft mod to a equivalent quadro using riva-tuner, but it won't be 100% like a quadro. From what i've seen the softmod on ge forces from series 8 would only gain performance with the custom drivers for 3d studio max...

By the way, the hardware is identical on a ge force or a quadro, the only difference is that on a ge force the professional functions are disabled at the drivers and some also are in the GPU.