Nvidia cards are very good, with excellent driver support. I've had very good luck with the ATI professional FireGL series cards and OGL (high end stuff like quad buffered stereo), but they are more expensive. 

I have a laptop (personal) with a Nvidia Geforce7600Go, and it performs quite well with P5, P6, Carrara Studio 5Pro, UVmapper, C4D, etc. At work I have an older (2 years) mid range ATI FireGL V3200 that has been rock solid, performs very well with Carrara 5 Pro, AutoCad Map 3D, Leica Geosystems Stereo Analyst and Virtual GIS, and Visual Nature Studio, but I don't use Poser at work. For my work applications, especially the quad buffered stereo, the ATI FireGL cards always seem to be a little bit better than the Nvidia Quadro series. At the consumer/gamer level cards I've only used Nvidia, and they have always been very stable.

Note: Poser, and other "low end" 3D applications don't use quad buffered stereo, and many of the "pro" level OGL functions/calls, so a good consumer/gaming card such as the 7xxx or higher series Nvidia cards will give nearly as good performance as the equvalant pro level cards. You probably won't notice any performance difference between a consumer card and an equivalant pro card with Poser, though benchmarks will show differences.

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