I was under the impression we were discussing gaming-level video cards with hardware texturing turned on. A Quadro offers nothing in Poser that a consumer-level gaming card can't. In the Poser world, an 8800 series is every bit as powerful as a Quadro or even a GTX series video card, as Poser makes only minimal usage of a GPU's capabilities.
I have not seen one shred of performance increase between my 8800 GTS and my GTX 285 in Poser.
In games, hell yeah, HUGE difference, but not in Poser and not in Lightwave or Maya either.

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Despite having two 8800GTX in SLI, Poser's default is to not use hardware shading in the preview render settings

SLI doesn't work in any windowed applications, unless they're written specifically to use SLI.
SLI and Crossfire technology only works for games. Applications will only use one of the video cards, and it might actually create a bottleneck as the PCI x16 bus speed might be slowed down when a second card is added.