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And you might consider an nVidia Quadro video card instead of the video game cards.  The Quadro is made for CGI work.

A Quadro will not likely perform any better in Poser than a top of the line gaming card, and it will more than likely degrade your performance somewhat in not only Poser, but games too, and maybe even 2D programs such as Photoshop CS4.
Quadros are made for CAD programs, where real time anti-aliasing is a factor, and for more advanced shaders that Poser can't even display.

There are very few programs that benefit from a Quadro, and they're all high end, such as Autocad and 3ds max Design.