The only thing that may be affected by a better graphics card is your preview (your work space). Your preview may look better with a better graphics card. I had a quite old one and now I’ve got a brand new one and the preview in D/S sure looks better. Also more memory on a graphic card allows you to display a larger scene properly. With my old 256mb card if I did create a large scene with a lot in it, the preview would be slow and at times textures would not be displayed. With the new 1Gb card this problem is gone. The preview/work space doesn’t slow down at all and no textures do disappear, everything works much smoother.
If you have the newest Poser, Poser Pro 2012, the a better graphics card will make a huge difference. It now supports hardware shading, a number of features that depend on the graphic card. It gives a much better preview of the scene, getting much closer to the final render. The better the graphics card, the better the preview will look. My new graphic card supports all these new features in Poser (my old one doesn’t) and the preview look awesome now. I know that a number of poser users create animations using the preview window instead of rendering and I can see them very happy, because their animations look much better now.
But for rendering as said, you’re better off buying a faster CPU and more RAM and use a 64 bit operating system.