Hmm, I'm wondering if it's not the scene itself? The other thing is that perhaps your cache sizes (L1 and L2) are affecting it. Honestly, I've changed out Nvidia and ATI vid cards with NO change in render time, at all. Bryce doesn't send any instructions to the GPU, only through it's wiring on the way out to your monitor, so that's probably NOT the factor. Did you use the exact same scene? Just curious, was it the same scene file? Just wanted to rule that one out...