Basically any one that lets you see your windows desktop or any other application you open.
Because Poser doesn't use the video card when it renders.  So if you  can see it, it
will work.   Now if you have Poser 6 you'd be better off with one that has Open GL,
but that it only for the preview mode, it doesn't use it when it renders.  And even then
you can disable Open GL in Poser and use a video card that doesn't support Open GL.
Myself I have an NVidia GeForce FX 5200, which while several years old, works just fine.
I have heard people have better results with NVidia than with ATI. So just find an
Nvidia board you like and go with that.  And make sure you have the most recent
drivers, which you can get off the NVidia website.